I usually add the second color in only about a row before I need it. You have to "strand" the unused yarn behind the one your working on. To do that- every three stitches, wrap the unused yarn around the yarn you're working with before you make a stitch. You should do this about every three stitches. When you're ready to knit with the second color, wrap the two yarn colors together (to strand them together so you won't have a hole), then pick up and knit with your second color. Be sure to strand across the back of your work whenever you have two or more colors involved.