As a former national team trapshooter, I would put 15,000+ rounds through my gun in a year for training alone. I know trap loads are far easier to deal with on a per-round basis than slugs, but when I would shoot 500 rounds a day during extensive training or shoot 250+ per day in competition I certainly learned a lot about how properly fitting the gun helped with the felt recoil.
First of all I'd suggest looking at the fit of the stock to your shoulder, and whether other stocks fit naturally better.
If you think the stock fit is good as-is, or if it's so ingrained into your shooting habits that you don't want to change stocks (or if money for a new custom is an issue) then drill holes transversely through your recoil pad. A few 1/4" holes in the pad will work wonders.
If money ISN'T an issue, get a shock-absorbing stock in which the comb and buttplate are fixed relative to your face, while the lower front portion of the stock can move on a bar system which is hydraulically or pneumatically damped. It's expensive as hell (relative to the other options) but it's absolute magic for a sore shoulder. I never had one personally, and didn't care for them really, but they're quite the setup.
-M
