If you haven't heard of them, segmented pad extenders (http://www.tothewoods.net/HomemadeGearSPE.html) have always worked great for me while using a pad. I'm not sure it sounds as though you need them though, if temperature was not a problem (they're more for cold spots around shoulders and such).
If you're sliding off center in the hammock, it sounds to me (though what do i know ) as if it's a problem with the hang itself. If the problem persists in the future, try to offset one side or the other as a preventative measure. So if you slide towards the foot, lower the head a bit.
Best of luck!