How to get a better canter?

My Thoroughbred is solid in his halt/walk/trot transitions, but his canter is all uncollected and sloppy. He'll canter from a trot on the lunge line, but when he comes back down to the trot, he rushes and he can't go back to the nice even pace right away. Under saddle, he can trot from a halt or vise versa easily. I've actually gotten him to canter from a halt a couple of times, and it's more collected and controlled then. But if he canters from a trot, it gets sloppy and rushed again. He's good about picking up the correct lead, he's just really messy about it.

I'm going to start him on side reins again next time I'm out there. Any advice is appreciated!

If your trot - canter and canter - trot transitions are not good, it is just like anything else to fix - perfect practice makes perfect. Make sure you are also not suprising him with your aids, count to three in your aiding instead of giving a quick sharp aid, tell the horse what you want, balance him and then ask him to do it, but also don't go let him get sloppier - he should accomplish the transition within 2-4 strides.
Lunging in side reins will also be a huge help, make sure the side reins are attached quite high on the surcingle (they should be horizontal when the horse is holding his neck correctly and moving uphill.)
When you feel he is unbalanced in a gait, use a circle to rebalance him. Make sure you are circling correctly and not allowing him to "motorcycle" around the circle. Use anything from a 12-15m circle in the trot and a 15-20m circle in the canter to rebalance.
Good luck!

That makes sense. Thanks!
One more thing: When you say "motorcycle", do you mean to not let his hindquarters drag behind and just follow the shoulders, but instead to keep the hind end around and working on the circle? Sorry, I've never heard that term before. Lol

When a motorcyclist turns, the bike doesn't stay vertical.

Avoid this!

That's what my girl is doing around corners. She's fine on the lunge line WITH a rider assisting her transitions. If I just canter around the arena she leans hard into the turns. Pulling up the inside rein seems to help some but not a lot. I've never heard the term motorcycle. I've always referred to it as barrel racing the corners.

Oh man. My thoroughbred did this too. >.>

It helps to do short canters. What I mean is to canter him half way around the arena, stop, back, walk off for a bit, canter, maybe trot, stop, turn, walk, canter, etc. It helped with the horses I've helped train because they don't anticipate the canter so much, and it keeps things changing constantly. As anebel said, small circles also work. Circles can be your best friend. ;)