Cast on, get it? (OK, sorry.)
Nothing is more aggravating than casting on using the long tail method and running out of yarn before you have the requisite number of stitches. As a general rule of thumb, you use 3 times the width of your project. For example, if your piece will measure 21″, use a 63″ tail of yarn to cast on. Better to have too much than too little. If your project will require seaming, consider adding an additional 12″ or so to use for that.