This is a nifty way to work in ends as you knit striped projects, or when you join a new ball of yarn.
Insert the needle tip into the next stitch on the left-hand needle, place the old color over the right needle (Figure 1; old color is dark), knit the stitch with the new color as usual, then lower the old color (Figure 2) and knit the following stitch as usual. Continue in this manner, repeating steps 1 and 2 and then knitting one stitch normally, for about and inch and a half to two inches. Cut the old color, leaving an inch or so of tail. After blocking, you can trim the tails to a half-inch.
Note: This method works best on small-gauge yarns. It tends to elongate the stitches a bit in stockinette, so it's best used with fingering- through DK-weight yarn.