My bra straps seem to always be falling off of my shoulders, so I decided to try a racerback style bra.  This bra had some great reviews so I bought this one and one other highly-reviewed racerback bra.  Both bras I purchased were very comfortable and fixed the strap issue.  This bra in particular provides a nice amount of support without being overly restrictive (I'm a 36B).  There is a small amount of lining in the bra that also provides for some modesty if the temperature gets a little nippy.  The other bra I bought at the same time is very thin and provides no such coverage (but is still quite comfy).