Actually, the reason people sleep on their back is because sleeping on your back can increase the chances of sleep paralysis, and the latter is the preferred way of achieving AP by many people. In my experience, getting out is the same in difficulty regardless of position.
To Blakey: Again, it depends on which method you are using to achieve astral projection. In some cases, it does not matter at all how you sleep. For example, if you wanted to astral project via lucid dreaming.
Based on your question though, I'm going to assume that sleep paralysis is the way you're trying to go. In this case, it's first important to know what sleep paralysis is. It's related to a safety function that your body executes every night, called REM atonia, so that you don't put yourself in danger by acting out your dreams while sleeping. With that being said, it should be cleared up that there is no "best" way to sleep. Your body automatically does this every single night. Sleep paralysis is when you wake up while this is happening. The goal is not to sleep a certain way. The goal is to do something that will keep your mind awake when your body goes into REM sleep and paralyzes itself.