You might know the situation: There's this gorgeous pattern your cannot resist to start right away. It requires 37 pattern repeats of 62 stitches. You happily start making a chain of 2,294 chains, carefully count them, work the first row --- and you run short of chains somewhere in the last crucial pattern repeat.
This is the point where I normally decide to abandon the project, however gorgeous the pattern.

But, if I still was into chain intense projects, there's my three options:

1. Count really really carefully (sometimes, it does work!)
2. Make more chains than you can possibly need and unravel the unused ones later (unraveling from the start is done stitch by stitch and with the help of a blunt yarn needle).
3. Take a second skein for the chains. Make (to get back to the example above) 70 chains, work the first pattern repeat, make another bunch of chains, work the second pattern repeat and so on. This kind of starting is especially useful if you just have an idea to explore, no pattern, and have no clue how many chains you will need in the end (or are too lazy to do the math).