I have a slighty different way of expressing the concept that eventdrew, JDI and others are referencing.
What you want is a *following* seat; where your seatbones slide in the saddle and follow the motion of the horse's back. This can also be a deep seat, but not necessarily. Here's some things the help to find the motion of the horse's back and the following seat. Find a model of a human skeleton and locate the seatbone. Then locate your own seat bones. This sounds goofy, but really, poke and prod around on your butt until you find the bony prominence and you know exactly where they are.
When you're in the saddle, make sure your seat bones point straight down. The position women seem to take naturally is to hollow their lower back and point the seat bones back towards the horse's hind feet, it takes a little effort to relax the back and point the seat bones down. This is tricky - you can have the correct alignment of shoulder, hip, heel and still be pointing your seat bones back.
Now, imagine that there are railroad tracks on your saddle, running right down the middle. Put your seatbones on the railroad tracks. When the horse canters, allow (*allow*, don't push) your seatbones to travel backwards and forwards on the railroad tracks.
When you're starting out, if you lose the motion and get popped out of the saddle, grasp the pommel of the saddle with your inside hand and pull yourself back into the saddle, and align your seatbones underneath again. When you feel yourself following railroad tracks again, release the pommel and see if you can keep the motion on your own. Grasp the pommel again when you lose the motion or your position.
Remember that the canter is three beats with a pause - the following motion is roughly: back, forward, more forward, hold, back, forward, more forward, hold.
HTH, and good luck!