Rule of 2 and 4. Take your outs (say, OESD).

Multiply by 2 to get % chance of improving on one street (8 * 2 = 16%).

Multiply by 4 to get % chance of improving on two streets (8 * 4 = 32%).

Works for any hand/draw.

What Sarbox said about implied odds is vital. You can't count on getting more money in after you hit, but it often happens. Plus, we stack opponents sometimes, too, especially with well-disguised draws.