* you should be able to fit two fingers under the band at the back and one in the middle centre front?
* if your bra rides up at the back then the back size is too big and you should choose a smaller back size or try tightening the bra?
* you should be able to drop your straps off your shoulder and the bust hardly moves?
* bra straps should not slip off your shoulder - if they do, the straps are too loose and should be tightened?
* the front of your bra and any wires should sit flat against your chest?
* your nipple should be in the middle of the cup and be sitting halway between the shoulder and the elbow?
If you didn't, maybe you should come in for a fitting? :-)