Don't wear flip flops. In the unlikely event of a serious incident during take off or landing, the last thing you want on your feet are a pair of flip flops. Wear trainers or shoes and, once at altitude, use the freebie slippers if more comfortable.

I am always flabbergasted at the lack of foresight so many passengers have (across all classes) that they could possibly think flip flops are sensible footwear in an emergency.