arghhhh. Tit Slings. they are the dangdest things ever invented. i am as a rule of thumb, 38 C. yet often they fit to tight, too loose, or not at all. i have some 36 C that fit well, as do some 40 C, so go figure. the worst ones are where the cups dont set where the girls do, so you get smooshed together or spread apart.
the best options ive found, is when bra shopping, do it in a store and try it on. nothing says " it fits", like the girls snuggling in and the straps not cutting groves into your back or sholders.
i dont do under wires. you get stuck once, that stops that.
a big plus in wearing/buying bras, find and stock up on them lil 'bra extender' things. they give the chest strap a schooch more length by adding in perhaps an inch or two of strap. hooks on one end, eyes on the other, 2 bucks at most stores for a 3 pack.
mostly these days, i opt for the sports style, stretch fabric, racer back design so no straps on the sholder sliding down the arm, no under wire pinches or mis positioned cups. they provide compression support that makes running and excersize comfortable, and all day wear at work a bit more bearable.
best advise, not trying to be redundant, try them on in the store. sans that, buy and take them home, and try them on BEFORE you remove the tags so they are returnable.