I would like to put my two cents in here: I have a boxer AND lab (well, shepherd cross) and they both do agility. What I found most important (aside from the recalling of course) was the attention.
If you clicker train, use a piece of food and hold it way out, at arms length. Then, bring it toward your forehead. When it's there, and your dog will obviously be looking at it, click immediately. Eventually, and after a few more steps, you can incorporate a cue... "look", etc.
I've found that if you make them look at you everytime something distracting is going on, eventually they look at you BECAUSE something distracting is going on! This will make it EXTREMELY easy when off-leash training comes around, because you'll know they aren't likely to go running after that loose dog who's dashing around madly. They just watch you.
Hope this helps!
rachel.