Analingus is the act of licking or inserting the tongue into the anus to provide pleasure. Both men and women enjoy analingus and you can use some of the same techniques on each.
Try to get into a position where the receiver's butt cheeks are naturally spread. Having them kneel on all fours (doggie style) with their legs spread apart, or on their back with their legs up in the air (again, legs spread apart). You may also use your - or their - hands to spread the cheeks and give you better access to the anus.
Different people like different techniques, so you're best off experimenting with several of those listed below to see which your partner enjoy's most. It's also good to remember that most people won't orgasm by anal sex alone, so you should keep analingus fairly brief or keep them stimulated in some other way at the same time.
Flicking: Gently flick the tip of the tongue across the outer rim of the anus.
Ringing the Bell: Push your tongue against the anus, then remove it, as though you were ringing a doorbell.
Licking: Use your full tongue to lick completely across the anus. This one may surprise you.
Probing: Insert the tongue as far into the anus as you can, then probe around with the tip. When you find their pleasure centers, probe some more.

Finally, it's not a good idea to perform oral sex or even kissing after analingus, as bacteria from the anus can wreak havoc on other parts of the body. Make sure to clean your mouth thoroughly after analingus (brush and use Listerine).