I'll put another vote in for the Gentle Leader. Very popular around here and is recommended by vets and trainers. Only downside is it may take the dog a while to get used to having it on and you don't leave it on.
Personally I have always used a choke chain with our boxers. They are a little "old school" but it's what I'm used to. In order for them to work though, they have to be in the proper place. High on the neck behind the ears is the only way they will feel it. If it sits low it will be on their shoulders and ineffective.
Whatever you do, don't use those retractable leashes. They confuse the dogs since they never know if they can run or if they should heal.