Sometimes, when my girlfriend is sound asleep, and facing away from me, I pull down her panties and rub my cock in her butt crack.  She's a very sound sleeper, and has a nice, big ass.  Her skin is so soft, and rubbing my cock against her smooth crack gets my precum flowing.  If I'm impatient, I'll spit on my fingers and rub it into her crack, and then slide my dick in there again.  

 Then I use my right hand to keep it in there while I grab her firm, natural C-cup breasts and caress her hips and pubic hair.  I have to be careful to keep the sloppy noises of my cock sliding in her ass crack down a bit, and avoid rubbing her clit or nipples too much, because sometimes she wakes up if she's overstimulated.  After a few minutes of slipping and sliding my member in her butt crack, I either ejaculate into my hand, or onto her back if she's laying on her stomach.  Then there is the simple matter of grabbing a few kleenex from the night stand and wiping my cum off of her and myself.

When my girlfriend is too tired for sex, masturbating in her ass cheeks gets the job done!