You'll want your off-ice warm up moves to mimic the moves you do on ice...and get the larger muscle groups pumping. Maybe some walking lunges, some squats, jumping jacks, etc. to get things rolling. It doesn't have to take a long time, maybe 5-10 minutes minimum and then some small stretches. Then skate an on-ice warm-up THEN stretch at the boards. If it is your hip flexor, you want to be extra cautious and take more time to warm it up and not overdo using it.