Reddit reviews Michel Roux Sauces: Revised and Updated Edition

The first secret of great sauces is great stock. Without it, you are nothing. With a nice batch of stock to hand, almost anything you throw in with it to call it a sauce will be lovely.

Beyond that, basic understanding of flavours along with a passable knowledge of the various ways of thickening a sauce to the desired consistency (e.g. simple reduction, roux, beurre manie, cornstarch, etc.) and when you should consider any of them appropriate should see you through. I heartily recommend Michel Roux's book on the subject if you really want to get down and dirty. It is mainly a recipe book, but it does a great job of educating the reader along the way too.