Finally, use your common sense. The rectal wall curves and is thin, be aware that long, hard objects can tear your insides. Don't wear any jewelry on your hands, and cut your fingernails if you are going to put your finger in someone's anus. Experiment, there are many more positions and techniques to try. Find the one that suits you best.
Good communication is the key to such discovery, and a manual or video can help as well.
Lubricants Liberal amounts of lubrication are necessary because the rectal wall can be quite sticky and is subject to tearing and lesions if not kept lubricated. Never use Vaseline or any other petroleum oil based lube. Use plenty of water soluble lubrication when you try anal sex. I recommend Ease, it has a spermacide that is believed to decrease HIV, or astrolube, although it tends to be a little light. It seems to stay around for a while.