Bra clips (which are available under a number of different names) are a plastic clip which will clip the bra straps together, holding them in the center of your back. http://www.thestrapper.com
When your bra style has the option for changing the straps, clear plastic bra straps work well. They stay in place and generally aren’t noticeable even when you are moving around.
Fashion Tape can also work to keep your bra straps in place.