The rule of 3 is a good starting place, though once you get used to it, breaking it once in awhile is really no big deal.

The rule of 3 is this - don't use big chainring with 3 biggest gears. And don't use small chainring with 3 smallest gears. As you are going up or down in the rear, use this to help guide you to know when to change chainrings.

55/Rad