If you're not "oriented" to be a bottom, it takes time. That said, the absolute best times I've ever had being fucked have usually been with me on my hands and knees in a sixty-nine position sucking on a nice tool while a third (and experienced -- "experienced" is important the key) person slowly pounds me doggy-style. Alternatively, kneel in front of the mattress or a couch, wherever you find the most comfortable bent over (and supported) position. Then you can focus on relaxing.
And the best advice? You should buy a set of butt plugs (usually three to a pack: small, medium, and not so large). Practice on yourself with lots of lube while jacking off. Eventually your sphincter won't be so timid and you'll learn how to open up and trust your partner.
Although messier than KY-type lubes, silicon-based will offer you the longest lasting and most lube for you're Euro or Buck. And they are condom friendly.
Good luck, and remember: most great bottoms are the most virile, masculine men around. :smile: