I ran across these TED videos about bra sizing, and I thought it would be of particular interest to those who have lost weight or are losing weight. The most important thing to know is that cup size is not independent of band size. For example, the cup of a 34 C bra is smaller than that of a 36 C. So if a bra doesn't fit well, you probably need to change both the band size and cup size in tandem. It's a very bizarre sizing system, and one of the first things I will change when I'm dictator.
Also, even if you're not losing weight, you should choose a bra that fits on the outermost hooks.That's because bras stretch, and before long you'll be using the innermost hooks.
Finally, the purpose of an underwire is not to provide support, but to give a certain shape to the breast. That may or may not be important to you, especially since underwires can be uncomfortable, or poke out in weird places!
While we're on the subject, here's something I posted on another thread a while back. I've posted it here for convenience:
Good quality bras are expensive, and if you're rapidly losing weight, you may not want to invest in bras that you may only be able to wear a few months. One option is to intentionally buy a bra that's a little too tight around the chest, and use a bra extender for a few weeks until you lose more weight and/or the bra stretches out a bit (which they all do).
Search on amazon or any lingerie shop for "bra extender". You can get them in 2-, 3, or 4-hook versions, and in a variety of colours. While they are a useful temporary solution for a slightly tight bra, I wouldn't use them long-term or with a bra that's more than just a bit too small. Better to invest in a properly fitting bra.