well for starters don't buy any of those creams or gels on the market that claim to give you uplift because they dont! the companies that make stuff like that are just making money by preying on women's insecurities.
you could work on improving your overall posture. look at yourself in the mirror sideways while you are slumped and then compare it with how you look when you are standing upright and straight with your shoulders back. it makes the most amazing difference.
the best thing for you to do really though, is to learn to love yourself the way you are! i have big boobs and so i know what you mean about the whole gravity thing! but people arent going to notice unless you draw attention to it.