Don't do generic conversations and don't look too much into facebook convos or msn or the likes, very different to real life.
I usually just try and find out a bit more about them and develop a bit of an 'injoke' if possible for the next time i meet them in real life.
Don't go on about yourself much, my main advice (in any walk of life really): Keep It Simple Stupid
p.s flirt subtly if you're interested in the girl. Little hints drive people crazy, girls pull that stuff on guys all the time. Innuendo and the likes.