Let's start at the top…or rather, at the bottom. A lot of things are going to be affected by how you size your drawers and a majority of your underwear problems are going to arise when you're rocking a size too big or too small.
Are the bands at the base of your legs too constricting? That's a clear indication that you need to size up. Are you tucking your underpants into your pants? You're probably a size too big. If there's one thing on your body that should fit you like a glove, it's your underwear.
If you're caught between sizes, determine where your sizing issues actually are. If the legs are too big, then size down. With waistbands made of elastic, a tight waistband on the first few wears will go away after the elastic stretches out. Feeling pinched in more ways than one? Size up. You already know that things shrink in the water, so even with an oversized wear, the size will shrink-to-fit after a run in the washing machine.