A BIG "second that" on Curbside Prophet's advice. We have a 5 m/o pup, a Great Pyr/Malamute mix, who is already around 50 lbs. We have been working on the loose leash walking for several weeks, but only in the past three weeks working on it correctly. I read the article mentioned above, and also started puppy classes, which teaches the same way, with some additional tips.
The women teaching our class are wonderful, and very modern in their teaching. We are using clickers and rewards, and Jack is doing much better on his leash walking. It does take time, and much practice. We practice several times a day, but not for a 15-20 minute walk. We focus on the loose leash for a good 5-10 minutes, then I let him relax and "wander", but NEVER let him pull me where he wants to go.
We don't get very far on our walks sometimes, and you should see us at Petsmart

, where I'm stopping and backing up, and then trying "forward" again. But it really is paying off.
Good Luck.