Provisional Cast On
There are a few provisional cast on methods, and you might see them named differently, but here are 3 that you might run into.
Crochet Provisional Cast on
This is the one I like to use, partly because it's the only one I can remember. With a smooth contrasting yarn, crochet a single chain a few stitches longer than you need. You can use a crochet hook, but I just use my finger. (Did you do this is as a kid? To start, wrap yarn twice around your right index finger. Holding the tail in your right hand and the working yarn in your left, lift the far loop over the first loop and off. *Wrap the yarn once around, and lift the far loop over and off, tighten the stitch. Rep from *.)
Pick up and knit into the back bump of each crocheted stitch until you have your cast on amount. When you are ready to use your provisional cast on, thread a needle through the provisional stitches, then pull the tail of the waste yarn away.
Invisible Provisional Cast on
Elizabeth Zimmermann describes this in Knitting Without Tears. It's very fast and easy. It doesn't look very tidy (at least it doesn't when I do it!), but once your pick up and work from it, it seems to sort itself out.
You can also work this one around a spare interchangeable cable; then all you have to do is attach the needle ends when you're ready to use your provisional cast on.
Chained Provisional Cast on
For this one, you need a crochet hook. You use one needle and the crochet hook to make a provisional cast on with waste yarn.