My girlfriend is 18 and I'm 31. She has never had anal sex but is willing to try it. I have done it a few times with other girls. I have fingered her anus a few times and it seems incredibly tight. I have quite a thick penis and am worried about it being really painful for her.  She likes some pain (like nipple clamps and spankings) but i don't want it to be too bad. other than her being drunk and using lots of lube, how can I help prepare her anus for penetration and relax her?