Made me think of an interesting point. When girls get conscious of bra straps showing (not that there is anything wrong with feeling as such), are you conscious about guys seeing it; other women seeing it; or just anybody seeing it?
I'd probably go with a convertible bra with a halter. Also, what is your bra size? Strapless don't fall down if you are in the right band size. They are almost the same as if you slip the straps off your regular. The band should not fall down or move when you do that.