Ear protection you'll be ringing for a while if you don't. I'm sure you've hear it before, but holding the gun (if it's a rifle) firm to your should makes a world of difference.

In my experience a higher caliber rifle kicks more then a shotgun. I've put a box of 3 1/2 12 ga BBBs through my 870 before and not had much of a problem, but i shoot shotguns quite a bit. I shot my friends Tikka T3 30-06 and definately felt that one more then the 12 gauge.