To keep the shoulders down, practice putting you body in the position you want to achieve, and then letting the shoulders feel heavy. Relax and really feel where the strain is, breathe, try to let the tension flow out of you - the physical tensions and the emotional tension too. You don't have to be anywhere near a dancefloor to do this - it can be waiting for the bus or whatever, subtly with your arms hanging at your sides you can still practice letting your shoulders relax down.
Plus when you get good at it, it's good for letting go of stress while waiting for the music to start in the competition.