Sometimes I'll take an old envelope, fold it, and run the folded envelope between the keys (left to right and back) to push the gunk off to the edges where I can pull it out by hand.
If you have a small (#1 or #2 Phillips) screwdriver, take the keyboard off and shake it/tap it against the desk. Most of the gunk will come out on its own. It's usually held on by one or two screws on the bottom of the case. Take the screws out and most lift straight up (some require a little forward pressure to slide out of place) after unplugging the ribbon.