I can;t get my dog in a down position...

If there is a resistance when you are pushing her back end down, don't use that method. Some dogs just don't feel comfortable with this kind of physical contact...
Get on the floor on your bum, and put one leg in an arc - a triangular arc. The dog is on one side of your leg, the treat is on another. She'll have to crawl under your leg in order to get the treat. When she lays down to crawl, use a clicker to mark the DOWN. Next, repeat again and use the word DOWN together with the action.
If this doesn't work out, let us know! []

Stick at it!  A great many trainers agree (yes! that's right! actually agree!  I know! Amazing!! []  that the "down" a very difficult command for most dogs to learn..... at one of the classes I went to I noticed that little dogs seems to struggle with it more, the length of their little legs makes it tricky. 
 
You can do as TinaK suggested, using the crook of your leg (or a handy piece of furniture), I've tried this and it works pretty well. 
 
OR get a clicker and shape the "down", CTing for every movement in the right direction and gradually raising the criteria of what the dog has to do to earn the Click....  So for example start the dog off in a Sit and you CT any time his head dips down.  When he's figured out that's what you want, click only for the lowest "dips", then for any forward paw movement, etc. until eventually you only CT when the dog is in the "down" position.  When the dog offers this position you add a verbal cue and begin only CTing anytime he "downs" on cue.  It sounds like a long winded way to achieve it but you'd be surprised how fast baby steps get you there.... and how much more reliable the command is when it's finally shaped.  The long windedness of it was what put me off at first.  I was pretty convinced that it was an inefficient and confusing approach but I was pleasantly surprised.  Aside from anything else it;s hugely pleasurable and satisfying to see those little cogs turning.....

What about training it in her crate? Dogs naturally lay down in their crate, so if you can catch it over and over and apply the "down" command to it, then try it outside the crate?

I agree with all the others.

I had to at first, take a treat, stick it up to Ella's nose and slowly pull it down to the ground. I would say down once.
I would have to work with her for about 5-10 minutes before she would down for me. Once she's down, a down stay is easy, but getting her into that initial down position takes a lot of work.

I still have to lure her into a down with a treat, I just don't have to hold it to her nose anymore.

Positive training takes SO much longer, but I've found it to be much more rewarding. The longer you work with your dog, I have found, the better the relationship between you and your dog.

you can place your pup in a down by putting your right hand on his collar and take your left hand put it behind your pups front legs and give the down command than sweep his feet out from under him. Once he is down hold him down for 30 seconds than you can give him a relese command.

I wouldn't use any of the physically place/force dog into a down. They sound like they are faster than waiting/luring/ positive methods, but they actually are counter-productive: many dogs find the Down position to be scary. If you start out by forcing them into it, they feel even worse about having to Down.  And you are setting your dog up to have a life-long aversion to Down, and when you most need the dog to Down, he will hesitate and/or refuse. Instead lure and take it slow and jackpoint reward that dog for downing, frequently, especially in distracting situations. The crawl-under-the-leg method is wonderful.

I can;t wait to try the crawl under the leg method. I am so very confident it will work. I have two dogs and have to wait until the one is asleep or resting so I can train with Sally. If I try to work with both at the same time, it is a disaster.

I stopped with the forced downs right away because for once, I was able to read doggy body language. I am not a dog expert in the least and I have trouble reading when my dogs are happy, anxious, frustrated, etc. But Sally made it very clear she didn;t want me touching her when training. She will take a light one finger pressure reminder for the sit, but anything more than one finger and she stiffins up, glares at me, and stubborness sets in. Hmmmm, I think she has my personality from when I was a teenager...lol. Anyhow, I would like to stay away from that stubborn side of her and won't be using physical direction, which my first dog respods to perfectly, and how our trainer taught us to train. Sally needs a lighter touch and more treats...lol. She wants her treats. In fact, I am feeding her her dinner while training because I am so worried about her gobbling down so many treats. So instead of treats...she gets her night time kibble. She enjoys it so much more than just getting a bowl of food. LOL, and I enjoy it too.

It worked perfectly! Like a charm! Thanks guys, I know sally is grateful!

Congrats!  I was going to suggest catching her laying down, click and treat. 
 
My dog knows that if I am snacking and if he has any chance on this earth to get a cracker or chip or piece of what I am eating he has to be lying down. 
 
When I use to eat around him and he starred or begged I would ignore him completely and then the minute he realized this he would lay down, THAT MINUTE - I would give him a taste of what I was eating.  I continued this of course for sometime and now when we are snacking he immediately goes in a down and waits to see if he will get a taste.  He knows there is no chance he will get anything if he is sitting or on all fours.
 
Good going.
 

I'm trying to remember how it was taught when Max when to obedience school.  I think that once he was in a "sit," I held the treat in front of him, backing up and holding it closer and closer to the ground ... and pretty soon he was down flat trying to get it. "Good boy."  "Good down."  Click. Treat.  He had it in just a few tries.
 
Joyce

The "leg bridge" method is what did it for Marlowe, just for posterity since the OP has already had success with it.  He was a dickens to get into a "down" for the longest time. I think it was an uncomfortable position for him at first (he has almost no body fat and laying on some surfaces can't be comfortable for him) and he had just decided that the cost-benefit analysis was not coming out on the side of laying down. So he didn't. Once I started feeding him all his meals from the "leg bridge" position, one kibble at a time, it totally clicked for him.

Well she learned fast! []
As a rule of thumb, when teaching any new command physical pressure can be totally fine as long as you feel no resistance. If resistance is there - you need to try something different.
I taught my dog some commands by physically pushing him down or rolling him over - he'd keep his body loose and watch me right in the face - he was letting me guide him, and help him show exactly what I want. But, now that he knows DOWN, he would never lay down if you push him.
There are folks who tell our dog repetitively "Down, down, down" while pushing him in that position - he looks at me, like - [>:] - "Tell him to leave me alone!"

I can't wait to try this with my pup........thanks for the great tip!

You said a lot when you said your dog is a JRT!!! These dogs are really great dogs, learn quickly and are a lot of fun. On the flip side, they are high energy as you know, always curious, always on the move and they want to learn everything! As another poster said, don't push your dog into a down position. The down for a dog is the most vunerable position there is and many dogs don't take to this at all.When I work with dogs who don't want to comply with the 'down', I have the dog on a lead (always during training so the dog doesn't wander off). If the dog is food motivated, I won't use regular treats for a non-compliant dog. I will start with regular treats (pea size, especially for a smaller dog) and if I don't get the response I asked for, I then use tastier treats. I never get down to a dog's level to train. I have my bait bag in the middle of my back and if the dog already knows 'focus', I call his name and if he looks at me, I praise, reach into my bait bag with BOTH hands and give him a treat. Using both hands is best because the dog is never sure where the treat is coming from and you want the dog to focus on you and not the treat.When I have his undivided attention, I begin the 'down'.There are two proven ways to teach this. You can 'lure' your dog into the 'down' with a treat in your closed hand which apparently isn't working for you or you can say your dog's name so he looks at you, toss the treat between his feet. When he finds it, praise him. Pick your praise word and never change it and say it only once. Say his name again and when he looks at you, toss a treat between his feet again. Praise.This is going to start him lowering his head to the ground looking for the treat. Because your dog isn't complying as quickly as you expect, you must use 'baby steps'. Other dogs 'get' this command right away and lay down usually following a closed hand containing a treat. If your dog actually lays down, probably accidentally, say "down". Wait until his belly touches the floor. I believe if you had some really tasty treats in your hand and you used the 'L' shape to lure your dog into the 'down', I think he would follow your hand to the floor. He will probably jump right back up because these dogs are like tight springs and can't wait to jump up. You have to teach him the down-stay. When I am working with a dog like this, I usually just stand in one spot, get the dog to look at  me when I call his name, praise then treat. When he realizes I have some really yummy treats like pieces of left over steak, and he is hungry, he will start looking for more. I toss the treats on the ground and when the dog eats them I say "GOOD". When he gets the idea that treats are 'found' between his feet, I lean forward a little and toss the treats between his legs, closer to his chest. He actually has to put his head down even more to get them. Most dogs, like I said, just lay down and eat the treats. Don't pull him by the leash to make him go down.Every time you 'catch' your dog in the down position, tell him "down" and toss him a treat, always between his legs or feet.At the end of your session, give your dog the 'jack pot' and toss all of your treats on the ground. Don't get frustrated while training your dog. This breed may be a little difficult to work with but once you get his attention and keep it, you will have yourself a really great dog that can easily be taught tricks and agility. Good luck!!!