How do you switch colors without cutting the yarn in crochet?

You can switch colors without cutting the yarn through a technique called "carrying along." In this technique, you add the new color just as you would if you were going to cut the yarn. Instead of cutting the first color of yarn, however, you place that yarn on the top of the previous row of stitches and work your new crochet stitches over it, hiding the first color with the new, second color.

Attached are links to illustrated tutorials which will show you this technique.