When your band rides up it is too big for you. Go for a smaller band and larger cup. Your band should be level with the ground all the way around your body.
If the closure is straining and your band feels uncomfortably tight you need a bigger band size. You may need to come down a cup size. You should be able to slide two fingers under your band.
When your boobs are spilling out over the tops or side (or both!) then your cups are too small. You need a bigger cup but not necessarily a bigger band.
When you lift your arms up your bra shouldn't move. If it does you either need a smaller band or a bigger cup.
Click here to see how to measure yourself at home.