If you hate estimating a tail or have an enormous number of stitches to cast on with a long tail cast on, try the two yarn method. This method ensures that you don't end up with too short of a tail by having one end or ball of yarn be the tail and the other end or ball of yarn be as it is normally.
You start with a slip knot to attach the two together on your needle, and then dip between the two different yarns to begin your longtail cast on. This method can be done in one color or two, and if being done in one color, the one end can be clipped off and the end woven in later. See our video on Double Ended Cast On for a visual walkthrough.