You probably know already to save some of that water when you drain freshly cooked pasta: It's full of starches that make a rich, flavorful sauce. What you may not know is that salty water packs quite a punch, so if your sauce needs some salt, add it in the form of your pasta water.
Image from D. Sharon Pruitt
The important tip here is to remember that the pasta water itself is salty, so you don't want to overseason your sauce. In fact, it's better a little underseasoned, so the pasta water can bring some salt to the party. When ladling in the pasta water, consider how much salt your sauce actually needs, since the water acts as additional seasoning.
If your sauce is already salty enough but you need that starch to make everything rich and smooth, you can still take advantage of the starchy pasta water as a binder, just dilute it with a little fresh water first, and you'll be good to go.