Take this all with a grain of salt, because I'm no expert, but I think this is how it would be.

When you exercise if you work the pectoral or chest muscles, as Heather pointed out, you can firm up the chest area, but to my knowledge this will not firm up your actual breasts. I mean, they are largely composed of fat, mammary glands, etc. that can't exactly be firmed through exercise. However, it can firm up the muscles in the area, which I hear can make your breasts seem a little perkier.

However, losing weight could have the opposite affect since you might lose fatty tissue from your breast area and your skin might not snap back. It might already be stretched a bit to accomodate the extra size and you might deplete the fat that is filling it up, causing extra droopiness the way sometimes there remains excess and now sagging skin when someone loses a whole lot of weight. I think it depends partly on a person's skin. My aunt had a baby and of course her skin had to stretch to accomodate it, but she has wonderful skin- has barely any lines on her face at middle age and I'm in my 20's with them starting on my forehead and lines around my mouth I consider too deep. So, when she had the baby she didn't get any stretch marks (whereas I never had a kid and was never THAT large- just a little chubby- and I have stretch marks in a lot of places) and her skin doesn't seem to sag at all in her tummy area, either. As for the skin on her breasts (since breasts also enlarge during pregnancy and usually go down afterwards as the milk production goes away and fat is lost, etc.), I have no idea how it looks as I have never seen her naked. He skin is elastic, so her tummy skin held up good. If you have good elasticity and your skin stretches and snaps back well, the potential droop factor might be less of a problem if you lose weight.

Also, you may or may not lose much fat in your breast area. If you don't lose much weight there, it shouldn't add to the drooping any, either. When you lose weight, I've heard the body starts off by taking fat from wherever it pleases. In other words, if I lose weight right now, even if I'm working my stomach out hard core, my body will start taking the weight off here, there, and everywhere it wants- NOT just around the tummy. Usually when I lose weight from not eating as much, I notice that my breasts do shrink in size, appear less full and less round, droopier, etc.- as if things weren't bad enough in that dept.- and I also notice that my face grows almost too thin (not fleshed out as I like it) while my tummy still has that dad-goned pooch when I sit down. Maybe it is more of a problem with muscle tone than fat, but I don't know- and there is a little fat left sometimes in areas. It seems like I can't find a balance. In order to get some areas of my body as slim as I'd like, I end up getting others too thin (such as my face and I hate the loss of breast tissue, especially thanks to the sag factor- I have terrible skin and breasts already sag) and even then sometimes certain areas have little fatty deposits. ARGH. Anyway, for me, yes I lose breast tissue, but I think some people's bodies burn fat more from other areas of the bodythan they do from the breasts, so it is prob. an individual issue and also might depend on how much weight you are going to be losing. Plus, I hear some people have dense breasts consisting more of other tissues than fat, so it might be less of a problem for them, as well.