In a bid to be more Gregarian I bought a white gel pen recently. It's nice for cutting back through black lines you've already put down and making negative shapes- I used it for that Entertainment poster, for example. So far I've only really used it on white paper though, and it's not very dramatic. Like pissing on yourself in the shower, most of the time.
Talking about pissing on yourself most of the time, here's an elaborate sketch of an old lady I did as a kind of practice. The hair in the bottom left is the closest to the kind of effect I'm getting at.