I know what you mean -- I feel permanently rounded from the weight and boobage (like baggage, only boobs). But there are some tricks you can do to help, like the ones mentioned by c laura. Also, try to keep your chin slightly elevated (greater than 90 degrees from neck) and back (and your shoulders pressed back and down so you're not hunching), this causes the weight of your skull to tip back past the spine, and the spine will fall into a more normal position to compensate. We spend a lot of time as human beings looking down -- down when we walk, down when we sit at the computer, etc, and that causes our cervical spine (neck bones and surrounding tissue) to straighten instead of curve back. The result is that bump you mentioned, and it can get worse with age if you don't take steps to keep it minimized.
Another thing you can do is work to keep the back and chest muscles flexible, since they work together in keeping the body properly aligned. If you lie down on a bench (or diagonally on a bed with your head at a corner so that your arms can fall naturally off the bed on either side of you), reach your arms out to each side with your palms up and let them fall naturally towrd the ground, keeping that palms-up position -- this will gently stretch out the muscles attaching the arm and shoulder to the chest, and help prevent the "curved in" appearance that we sometimes get from the pull of boobage on our front side. Don't push it though -- just let yourself gradually feel the muscles stretch. If you're doing it right, it feels pretty good.
