Ditto the above. Try a cheap pad first. Limbsaver is supposed to be pretty good. Go to the Knoxx website, there's some videos about how it helps (showing shooters). There's also the option of a recoil pad that mounts to the stock, in place of the standard pad, that's supposed to be good too (I think Limbsaver makes them as well).

Personally, I found that my 870 doesn't kick too bad with buckshot. Slugs, yeah, they hurt. But then, I'm used to a Mauser and Mosin-Nagant, so I'm kind of a "big kick" kind of shooter. If you're using birdshot, it kicks even less (for home distances, birdshot MIGHT be OK). Personally, my first round is bird, with follow up buckshot.

FYI, I have a slip on pad on my 870.