Superman was well known for his superpowers and one of his most famous was his heat vision. He can cut through metal, fry bad guys, and spot weld broken machinery. You can practice your heat vision pose by staring intensely at a fixed point!
As a non-kryptonite it's going to be hard to replicate Superman's super speed. You can, however, practice your starting stance. Hold your arms steady at your side like you are about to take off running after a nefarious thief. All of your fans will be so impressed they'll forget that you're not the real deal!
One of Superman's greatest traits was his super-strength. He was of course able to perform amazing feats that no human could replicate. You can augment your costume by taking stoic stances. Flexing your muscles will assure everyone of your Superman authenticity. Even if your physique is not particularly muscular you can still practice these stances for dramatic effect!
Superman of course used the power of flight to fight his foes and traverse great distances. While donning a Superman Costume won't give you the power of flight, you can practice your up, up, and away pose. Just put one fist in the sky, one at your side, and utter the iconic phrase!