My ex-gf used to play with it a lot and do things like writing letters. Sometimes it gets oversensitive though, and I like a lot of slow strokes. Not sure what my boyfriend does, he kinda does a bit of everything depending on what noises I make.
it is much easier to go side to side with the tongue. also, i think it is important to watch where a women stimulates her clit when she masturbates. it typically is in the same spot every time. remember that spot and lick there.
I actually really like circular movement. left to right gets me off pretty quickly, although its nice when its up and down especially if they're going underneath the foreskin of my clitoris (but its super sensitive)