The other day my girlfriend said that in the event of us moving in together (which may be on the cards at some point), she'd be comfortable doing up to about once a week, which suits me as I want it to retain that taboo feel, which I think it might lose if it ended up becoming something we did every other night.
We've found that the optimal way to do it is for her to lie on her front with a pillow under her hips and to use her small 'bullet' vibrator on her clit as I fuck her from behind while gently probing her arsehole with my finger and some lube. Then when she's ready I enter her that way very slowly, taking several minutes to get about halfway in. By that time she's really into it and I can move slowly in and out. Then once she's come I can gently get her up on all fours and fuck her until I come insider her, which predictably doesn't take very long.
Interestingly she said that when she comes that way, her orgasms seems to start from her g-spot, as it does when she comes while I'm fucking her and she's got no direct clitoral stimulation. Whereas when she's rubbing or vibing her clit while I'm fucking her the normal way, it seems to start mainly from her there. Just goes to show how closely 'wired up' all that anatomy is, I guess.