Another important thing for being penetrated, (and I wish I had known this the first time anyone did it to me,) is to 'push' like you are trying to, well, you know.... Since one should have already douched, (very important, using ONLY water in there,) there is no chance of anything coming out, but you will be opening the sphincter and making it easier for the penis to enter. By the way, douching is good for cleanlyness, obviously, but also good to make sure there is more room for a penis inside. A penis hitting waste inside someone can be very painful for the bottom.
Another thing is that it is generally best for the first time for the receptive partner to be on top and totally in control of the penis entering until the anus is totally comfortable. Having a penis forced into an anus, no matter how tender the top might be, can be very painful, while one entering this way, with the bottom expanding his anus and controlling the initial entrance, can be incredibly pleasurable.
As for having a top rim an ass before fucking it, that is only common curtasy, in my opinion, not to mention incredibly hot and pleasurable for the bottom.