OK, maybe it's a little early to be bragging about this (you never know!) but I wanted to let folks know that Juneau finally seems to be getting the whole loose-leash walking thing down - all trained using only a clicker, treats, and a flat collar (never used a choke, harness, or headcollar). This is a Samoyed - a sled dog - so it used to be like racing the Iditarod to take her for walks. Over the last 3 days or so, something has "clicked" and she is now almost always - 90% of the time - just with me on the walks and not pulling ahead. Amazing. Of course, it has taken us about 2 months to get to this point, so it was not an overnight success!
It has taken a LOT of patience, repetition, and most of all consistency. I think there is a tendency to tweak the training technique if something doesn't seem to be working one day - but I stuck with it and have had a little breakthrough recently. Now when we are on walks, she is really focused on me, and will even stay near my side when she gets distracted by other things (cars going by, leaves, a piece of trash on the sidewalk, etc.) Even better, in those cases she does pull ahead she will come right back to my side once she realizes she's gone too far. She is not perfect, of course, but it's incredible to see her being able to generalize.
I want to recap the approach I took in case other folks find this useful - would also appreciate any tips/advice for improvement! Also, I wanted to thank the folks in this forum, since it was your encouragement to try training the loose leash using only +R methods that led us down this path - I think my original post was asking how early would it make sense to start a pup on a choke collar, but the "clicker posse" came out of the woodwork to show me the light

Thanks everyone!
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My technique is pretty simple. Start out with a sit. J-dog on my right side (I'm left-handed, so hold the leash in my left hand with a treat in my right). Hold the treat in front of her nose. Then I say, "stay with me" as I start walking forward, keeping the treat close to her. She will basically just keep her eyes on the treat as we start walking - often getting a foot or two ahead of me and looking BACK at the treat (not where she's going, so she will trip over the curb if we're crossing the street!) As we're walking I'm praising her over and over: "good dog!" Then, after a few steps, I stop. She will sit automatically and I click/treat at that point. (Before I had to lure her into the sit or ask specifically for it, but now she understands the routine.)
The value of the treats matters a LOT. We used to use kibble and got very little response (although kibble worked great as a training treat at home). We switched to Natural Balance rolls, cut up into little chunks and stashed in the fridge - this made a HUGE difference. They are really stinky and yummy (I even tried a bit and it's actually pretty good

Just buy the large "dog food rolls" not the "training rolls". The training rolls are, as far as I can tell, the same stuff, but cost 3x more since they are packaged as "training treats" instead of "food".
She used to immediately dart off as soon as I gave her the treat, but now I say "stay" and grab another treat from my pocket quickly, so we start all over again before she gets distracted. This has helped a LOT since it means we can just repeat over and over.
Increase the length of time we have to walk together before clicking and treating. I tend to vary this on our walks - sometimes I walk only a few steps before c/t, at other times the length of a whole block (amazing!). I think setting her up for success is important: sometimes making it really easy for her to earn the treats helps to reinforce the behavior.
After MANY walks and MANY repetitions she finally seems to understand what I'm asking her to do. If she pulls ahead on the leash, I stop in my tracks and she will come back to me and sit. (Sometimes I call her back to me.) Then I start over - treat in front of her nose, "stay with me", etc.
The times she is not so good are when we are within a block of the dog park - she knows EXACTLY where we are headed and wants to get there as quickly as possible. I am trying to avoid letting her pull me the rest of the way, and instead psyche her out a bit by turning around and walking the other direction, which causes her to snap right back to my side and watch the treat, since she has no idea why we're going the wrong way. I lead her to the park gradually by walking 20-30 feet towards the park, then turn around and walk 10 feet in the other direction when she starts to pull. It takes a while, and I get kind of dizzy doing it, but I think it helps to reinforce that she needs to stick next to my side.
Sorry for the long post but hope some folks find this useful!