To start with the best positions are either doggie or bent over an object. It's easy for your partner to get at your anus, and you can relax.
Another good one is lying on your back with your legs up. This allows for plenty of eye contact and makes it easy for her to get at your penis. The downside is that you have to hold your legs up, but you can come up with a device that uses straps and goes around your shoulders to help, or you could get something like this: link