I haven't read what's been proffered before, but I'll just say my peace.
If you wanna do her up the ol' dookie chute or vice versa, I suggest going with a smaller sized, round (not penis shaped) vibrater first.
There are two reasons for this: 1)a small vibrater feels much better than a penis in the fact it vibrates and that it is very smooth 2)if you just jamb your dick in there, she won't wanna let you stick anything up there for a while (fucking hurts).
I did the whole jamb your dick up there two years ago. It's taken me this long to get her to agree to it again, so take heed!
It is much easier if you let her start out by sticking the vibrater up your ass first. Yes, it sounds a little gay, but it really doesn't have to be. Ask her to try it on you (lots of lube), and later try it on her. Women are much more receptive when you've tried it first, and a small vibrator up your butt isn't bad. If they hit the right spot it really isn't bad at all.
Anyway, try this and let us know how it goes.