the best way I learned to teach loose leash walking, is with NO leash at all. exactly as lvoss said… when the dog is next to you, REWARD. I did this in a well fenced area, so I did not use the long lead as a safety-line, but it's definitely a great idea when not in a secure area.
I also use the Black Dog head harness instead of a Gentle Leader (I must admit, I LOL at the "genital leader"). I found the Black Dog harness fit MUCH better, and did not ride up into the eyes.
another product I've used with much success is the Easy Walk harness. it's alot like wearing a shirt that is too tight - it would be hard to lift your arms all the way up. this works the same way, it's tight across the chest/shoulder area, so the dog can't get much ground on you, making it hard to pull.
I still think the best method is what lvoss described, rewarding good walking skills. The harnesses do help for the times when you need to walk the dog, especially in public, and your training isn't yet perfected.