Betting on a draw

I've been reading Harrington's tournament books, but I get a little confused when he talks about betting when you have a draw.

He talks about "the no-limit semi-bluff" when you ie. have a flush draw and bet on it. He also says you chould not bet on a draw, because you give your opponent an chanse to chase you out of the pot.

So when do you guys think you should bet on a draw?
What factors should you consider?

Thanks!