Ok, I promised some bra fitting tips, so let's start with "how to tell if it DOESN'T fit right" - I'm only going to talk about things I've encountered myself:
Issue: The center front of the bra is not laying flat against my chest.
Answer: Either the cups are too small OR the bra is not curving back in toward your chest enough as it approaches center front. To tell which is the problem:
1. Is the bra leaving an impression, particularly from the seamline, on you after wearing it for 15 minutes to 1 hour? Or is it causing your girls to hurt (because they're being squashed!)? If so, the cups are too small and you need to make them larger. Measure the gap between the bra and your chest at center front, and for every 1/2" of gap, go up a cup size in the bra pattern you have and try that size.
2. If the bra is not leaving an indentation on you (or hurting your girls), then most likely both the upper cup and the lower cup need to have a tighter curve back toward you as they approach center front. Think of where the two pieces meet as a dart - the dart is not large enough to cup back inward toward center front. To determine how much to curve these, undo only the center cup seam from center front toward the apex, stopping before reaching the apex. Also undo the seam between center front and the upper cup. Now with the bra on, pull the upper cup down toward the lower cup so that they overlap. When the cup conforms to your curve from apex to center front, you have pulled the upper cup down enough. Pin in place and then draw with a fabric marker where you want the seam to lie on the pieces. Change your pattern accordingly, sew this, and see if the problem is corrected. If so, then try wearing it for a day before changing anything else.
I will add more tips later. Gotta go sew for now!