The answers you're looking for are lube, lube, more lube, alcohol, and dating women who have already had a child... But seriously, lube, time, and a drink or two to relax her. Oh, and stop to put more lube on. Don't use positions that give the greatest penetration depth as well. IE: Don't do doggy style and if you do, be gentle about it.
The other thing that can help is putting 10 lbs or so on, it'll add a little padding around the base of your tallywhacker. I think the average female vaginal depth is something like 4.5" and like penises on men, they're all different. You could just go in for dating size queens I guess.
Me? I'm hung like a can of tuna... (No, not really, I just don't really talk about my endowment on the internets.)
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."