In addition to copious amounts of lube, gently finger her ass to loosen her up. Then press your cock in slowly, making sure to go in perfectly straight (it hurts when the angle is wrong, but if you do it perfectly, there'll be no pain). As you push, tell her to push out like she's taking a dump; that helps a lot too.
I'm a very experienced bottom and these are things I try to do every time I get fucked.