I am in agreement with everyone on this thread...however, I must say that my dog refuses to walk with the gentle leader. You would think I was prodding him with hot pokers when I try to put it on and walk him!
What has worked well for me is using the fact that a walk is perceived as a reward to your dog, and they live to make you happy. So as soon as you step out the door, have them sit at your side. Use a trigger word when you are ready, such as Ok, let's go, heel, whatever works at first. As soon as they start - walk ahead of you instead of at your side; stop, and tell them to sit. When they are situated again sitting at your side, again use your trigger word to start walking. Continue to repeat this, they will start to realize that you are walking them, not the other way and will heel at your side. The reward is a noticably happier walker and a pup that gets their walk - albeit with a lot of stops

As a by-note, pulling on a leash often has a lot to do with energy. Boxers have tons of it and they want to run and play. So walking a tired boxer is much more manageable than an energetic one. Plus they will listen and learn commands much better when tired.
You will find that heeling when walking is as important as "come" is to off leash. Good luck and enjoy!