Try the silicone based products. They don't dry out and gives the ass that o so smooth feel. It makes it a lot easier on the ladies if it's lubed properly. Not sloppy, properly. These silicone products, Wet I believe it's called, are far superior to the old KY, or whatever is being used. And on her back seems to be the concensus for comfort. Once you get past the initial penetration, just take it slow. Don't hurt them, or you'll never be invited back!