Primarily, you have to get loosened up right. Let the Top finger you nice and slowly. Start with rimming if the Top likes that. When you are squirming, wet and ready have him start to finger you. He should start off with one finger going slowly until you are adjusted to the single finger, and it is feeling really good. Once you are totally relaxed and enjoying the single finger working your ass, the Top should then ease a second finger in slowly. Have him work it until again it starts to feel really good. After you get to the point were 2 fingers feel good, follow the same steps with 3 fingers, unless the top is kinda small. You have to keep going, with the Top adding fingers to the point where the number of fingers basically matches the Top's dick girth. By this time you should be damn hot, and wanting that cock sliding into you, and if not then you may just not be a bottom.

I find the best position to take a cock comfortably is while lying on my side with the top behind me. Straddling the top while he is on his back can be good too, since you are controlling the penetration. I think if the top is good and knows what he is doing you should probably be able to take his cock comfortably whatever position he wants to penetrate you.

Lots of good lube is important too. The silicon based lubes are my favorite. They are very slick, not sticky and don't really dry out during use.