Make sure she's alive?
No really...
Going down on a girl properly is as complicated as it gets when it comes to sex. Each girl is so different that you can find a badass move that works on one and it will just piss off the next. You just have to be aware of the girls movements and if you can hear with her legs wrapped aropund your head noises are a good sign that your doing your job... And last but not least don't be affraid to ask a girl if it's going alright.
Oh, and remember to whipe off your mouth before you go in for a kiss... some girls don't like to taste themselves.