My internet is REALLY slow today so let's see if 3FC lets me actually post this!
For starters, make sure you're wearing the correct bra size. The best starting place is taking your underbust=band size. I would link to the fit page on my blog but the internet won't let me do much at the moment but if you need help go to my blog in my signature and click on "Finding Bras that Fit", which can help you out with measuring/fit.
Swimsuit are a bit tricky, especially if you're going for one-pieces! I have a long torso and a big butt but slim hips. I have yet to find a one-piece that fits correctly and doesn't just give me a constant wedgie.
That being said I have had some great success getting alterations done to turn a one-piece into a bikini so I still got use out of it.
What are you planning to do with your swimwear? Is it for swimming laps/water aerobics? Because if that's the case I'd recommend the Freya Active swimwear (they have a tankini) and I've heard amazing things about them. In that brand they run tight in the back so I do fine going up one band size and down a cup size (I wear a 28HH/J normally and my Frey Active size is a 30GG/H... the 30GG works well for me but I'd probably need a 30H for a better fit).
If you're looking for more casual wear then there are a lot of great brands out there that size by cup size and go up to the higher end of the alphabet. Not sure what your size is but if you let me know I can help you with brands/fit. Depending on the swimsuit you might want to go up a band size but not necessarily. It's a bit confusing, I have to admit, but there are a TON of bloggers out there who have reviewed swimwear so often you can find reviews to get help with sizing on a specific swimsuit/tankini/bikini.
ETA: Also check out Elomi. They just came out with a swimline and I actually really like their concept for tankini's+high waisted bottoms.