The instructions for Great Stuff used to cover this. Remember when it first
came out and a folded booklet was inside the cap of each can, along with a pair
of disposable gloves?

Anyway, when you are done, unscrew the parts. Dampen some flat wooden
toothpicks and shove one in each of the holes, large end first. So that's one
toothpick into the can, and one in each end of the plastic nozzle/trigger. The
foam will harden around the toothpicks and it just pulls right out later.

It doesn't work as well for the tube since it is too long, so I give it a
squirt of lacquer thinner or acetone when I am done. This dissolves the sticky
foam. You can also clean the parts and can with the solvent, but if you leave
even a little bit it will jam, so the toothpicks work better.

--
Dennis