Depends on what you want Oo
You can make good effects with only black and white or you can use different kinds of hatching: Cross hatching, like @insomniascrawl said, but also only one row of lines at the same time. I use several techniques in my manga Remember. See here, the smoke:
- Smoke: lines with change of direction, framed by thicker putlines
- upper left: Crosshatch, good for gradients if you play with thickness, length and space of the lines
- bottom panel: Speedlines, roughly all the same direction
- Also upper left panel: A lot of black helps to create the effect of a strong light source (like the many black on the child there)
My advice is: Try out. Try copying existing images and then draw it your way. Basically: No smoke in the world looks like I've drawn it. But I like the effects, so I decided to go with that. =)
And most importans: HAVE FUN!
I hope I could help, if you have more questions, please feel free to ask, I'd be glad to help further. =)