It's funny, the better you ride "western" the more Dressage you use, just with variation. Working on your balance through circles, lots of leg-yielding, half-halts, etc. will all aide in slowing your horse down.
To slow down the canter depends how slow you want to go, and if your horse is physically cut out for it. Softness is always key. Lots of lateral flexion and big releases when you are teaching collection. Working through the butt, two-tracking, half-passing, pivoting, rollbacks, etc. Fast and slow work through transitions.
I use my knees to half-halt. If I press in my knees, my horse knows to slow down. I started at the walk, pressing them and backing up. Soon he associates that pressure with backing. You can get a nice subtle crawly walk and a slower rate of speed for the jog and canter with enough practice.
But to rate the lope, I usually two-track, circle, or back them up before loping.