how can I stop my dog from eating things in our back yard?

I am in a sense going throught the same thing with our new puppy Bear that you are going through.  The only thing that I can think of that may work that I myself and working on is training him command words.  For example, If Bear starts to eat or pick up something that I don't want him to have I will call his name in a higher tone and say "Bear stop".  It also works well if I whistle at him being that it really gets his attention.  I try not to use the word "no" being that I have read in some books and seen posts on this board that it is not such a desirable word to use when training your dog. It's a slow process for me so far being that our pup is only 7 weeks old but maybe it will work better for you being that he is a bit older. Good Luck!

SEVEN weeks old???  That baby should still be with his momma and littermates.[]
 
OK, back to the topic at hand......go outside WITH your pup.  Teach him LEAVE IT.  Don't let him have free acess to the yard until he is reliable with leaving whatever you tell him to leave.  Personally, I'd leash him for outside trips for now.  As you've already seen, eating stuff he finds in the yard can be detrimental to his health, and a reliable "leave it" could save his life.

hmm. well, I actually consider grass, twigs, and rabbit/deer droppings to be fairly healthy additions to a dog's diet. But, really, a 6-month-old puppy should never be out there alone. You need to be out there with her, guiding her behavior, and making sure she doesn't develop nasty habits like digging craters.

We really don't let our dog outside for any extended period of time alone. I mean, our yard is fully fenced and safe and whatnot, but the fact is - she doesn't do anything to entertain or exercise herself out there unless we are with her. So she goes out for a pee/poo for a couple minutes, and then back inside. When it's time to play, we go out with her and can then use "leave it" when she gets into anything we don't like.

"No" is a swear word in dog training, not of itself but because of how it is used.  We tend to use it all the time without even thinking about it and too many people use it for a wide variety of different things that they expect the dog NOT to do, which is confusing for the dog.  In addition, they often haven't taught the dog what it means FIRST before using it for real, expecting the dog to learn "on the job" as it were, which sets you and the dog up for failure because there is such a high chance he will get it wrong.  If you are not careful you can fall into the same trap with "stop" or "don't" or "leave it" 
 
Plus *and this is the most important bit) dogs (ESPECIALLY PUPPIES!!!!) do not need to be told what not to do.  They need to be told what TO do.  Go outside with your puppy on a long line and keep him away from what he is not meant to have.  Have a favourite toy or some treats and play some exciting games interspersed with training.  If the dog is going out to pee/poo use a shorter lead and wait for him to go, when he has "been" then give him more freedom but always, always, always, make yourself much more interesting than rubbishy twigs and grass.  Regularly sweeping the area he uses to clear it of droppings will also help - nothing better than droppings for a lot of dogs and they are hard to compete with!  Plus, preventing the habit from starting/becoming ingrained is much easier than trying to cure it later.