If it's going to have any hope of working, you need to have something thermally conductive between the heatsink and the motor. If you have another method for securing the heatsink to the motor, buy a packet of heatsink compound from your local Fry's or Radio Shack.
If you want to take care of it all at once, you can use thermal epoxy.
Here's a link to some cheap epoxy:
Thermal Epoxy
If you want it to be non-permanent, mix up the epoxy, then mix in about 33-50% plain old heatsink compound.
Chances are any heatsink compound is a bit of overkill, but if you are actually needing to worry about heatsinks, then you might as well make sure they work as well as possible, and it doesn't cost that much.
----------------
If we're worrying about extreme heat, Artic Alumina Thermal Adhesive says it's rated to over 150C I'd be worried about a motor that got hotter than that.
Arctic Alumina