Jeni,
I use the Martingale collar for Cody and his walking nicely at my side has really improved, but earlier on he was one that lagged behind rather than pulled; On occasion, he will now pull, but I do the "make like a tree" also which he has learned that I will not budge if he pulls. He has come a long way over this past year. Earlier on, I had tried a harness with Cody and it did not work and I even wrapped his leash around his mid section as my instructor once told me to do, and I think Corgimum posted a picture of how she did that with Dylan. When I did that with Cody, he rolled over on his back and would not get up.
But the gentle leader sounds like something you might want to try with Dudley.