Frisket
If you need to cover large areas of your painting, frisket is probably your best option. It's basically a sheet of clear plastic with an adhesive on one side. You can lay it on your work and then cut out the material over where you want paint to go. You can then peel off the material at the end to reveal clean lines.
Masking Fluid
For smaller areas and those where you want a more painterly effect in your painting, masking fluid might work. It is a liquid that can be painted on with a brush and can make detailed areas where your painting won't be affected. Simply rub off the dried fluid with a kneaded eraser.
Masking Tape
If you've got straight lines to mask or areas too small to use frisket, you can cut up small pieces of masking paper and place it over the part of your painting you don't want affected. You should be careful, though, if the tape is not firmly pressed onto the working surface, as small amounts of paint can bleed under.
Tracing Paper
An inexpensive alternative to frisket is tracing paper. You can apply a spray adhesive to one side, then attach it to the painting. Just like frisket, you can trace out what you want to cut out. Once again, you need to be careful with this technique or some paint bleeding may occur.
Movable Masking
If you're using an airbrush or spray paint, constantly masking your painting over and over can be rather daunting. With a little practice, you can become proficient at holding a sheet of paper, sometimes cut to particular shapes, in between the spray and the canvas.