Kick killer or PAST strap on shoulder pads aren't bad. Slip on pads are "one per long gun", whereas the shoulder pads are universal. The disadvantage is that it's another thing to put on, and a slip on pad is ALWAYS in the right spot, whereas you might miss a shoulder pad if you're not used to wearing it (not positioned right).
For slip on or screw on butt pads, I'm a fan of pachmayr decelerator pads.