Get your horse responsive to your aids, he should have an immediate reaction to any of your aids. Once you have established that, make sure your horse clearly understands what you mean when you ask for a canter. Relax, and go with it. If it works, it works, if it doesn't try again and hopefully you'll be able to learn from it!
if you're practicing transitions, try to test if your half halt "goes through", if you have the feeling "yes, I could now go to walk if I seriously asked for it" go ahead and do it, but if you feel "it wouldn't really work now" don't force it but really send him forward and "test" again. That way you'll not be practicing crappy transitions and get a more responsive horse.
Try to just do a more transitions at slower gates and also take the time to make them smoother so your horse can be more responsive when you do ask him for the transition he will respond better and have it be smoother