Here are a few tricks...
1. Dust all loose crumbs off the cake before frosting it. (A pastry brush works great!)
2. Make sure the cake is cooled completely.
3. Stick the cake in the freezer for 30 minutes before frosting it. (I have been known to let it freeze overnight and then frost it the next morning. I haven't had any problems with cake flavor or texture. Just don't freeze it after frosting, because it will sweat.)
4. Don't be afraid to use a lot of frosting. Put a clump of frosting on the cake and then smooth it out. (Not little bits at a time!)
5. Wipe your spatula if crumbs get on it! (Hint: Using the right kind of spatula will help too. I recommend the smaller metal ones like the one below.)
Have fun!