Go get a proper bra fitting. It's intimidating and weird, initially. (Another woman looking at my chest?! HORRORS!) It's also so completely and totally worth it.
When I went for my fitting last June, I was wearing 44DDs. (The largest off-the-rack size at Wal-Mart.) I am embarrassed by how wrong I was on my size. (I now wear a 42 DDDD.
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One of the things a wrong-size bra does to you is put most of the pressure for support on your shoulders. According to my bra fitter, about 90% of a bra's support should come from the band, not the straps. I'd still like to lose a few cup sizes, but my neck and shoulders don't ache from carrying my boob-weight all by their lonesome any more. (I also look a lot better -- and thinner -- since my boobs are up where they should be.
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Another thing I've learned - those of us with extra in the chestal region can't wear halter-style bras. Again, it places the burden of support on your neck, which is not-so-great if you've got any weight in the chest area. (Halter tops are a different story altogether.)
Hope that helps and wasn't WAY TMI.