I was actually going to post a thread about this myself, as I am seeking the same advice. I have 34A breasts, and sometimes when I'm not on my period or ovulating (when breasts sometimes get bigger or fuller), I even have a hard time filling that out, especially if there is little or no padding. People always used to tell me, "Just be patient, they'll grow." Well, I'm 20 years old now, and they haven't changed at all since middle school, and I don't see them ever changing in the near or distant future.
I don't want kids, now or ever, and even if I did, I don't know if I would/could breastfeed. The point, though, is that I don't want larger breasts for the sake of having children, and I don't even want bigger breasts simply because it's considered a beauty fad of society. The only reason I want bigger breasts is to simply feel like a woman. I already look very youthful, and that's fine. I like looking and being young, but I don't want to look like a prepubescent teenager. In other words, I don't want to look that young. I am an adult woman, but it's hard to feel that way sometimes when I see my breasts are as small as they were in middle school.
Another issue I have is that I can never go without a bra if I don't want to look absolutely flat chested. I even sleep with a bra, just because I'm so used to wearing one all the time, and since my boobs are so small and my bras are usually comfortable, it doesn't bother me. Bathing suits are an even bigger nightmare because they sometimes have no padding, or just make me look really flat. It makes me feel really uncomfortable and like a little kid. I don't even want big boobs, I would actually be comfortable with a B or C cup. I'm too thin too have large breasts, and I really don't need the back issues. I'd just like a little more breast to feel more like a woman and less like a child.
I would never do implants, however. I'm not that desperate or willing to put my health at risk and cut up my body. A lot of guys try to tell me they don't think I need bigger boobs because the ones I have fit my body type, but that doesn't really make me feel better about them. So I try to live with them the best I can and not dwell on it so much. If anyone has the same issue and has found something that has helped them overcome these feelings (without drastic measures like surgery or something), please share.