I'm a 36C, and I buy the convertable bras from victorias secret. They can be worn with or without straps, and they are the only strapless bras I have ever found that actually STAY where they belong without having to pull them up and adjust them all the time.

The other thing I buy is those tanktops that have the built in bras or shelf bras. The tank tops have straps, but the straps of the tank top are not like bra straps (they do not pull on my shoulders or cause any pressure).