The two most important things for me to learn were 1) Work primarily belt-to-car and 2) work in the direction of the belt.
Always start at the front of your work area and work your way to the end. That will minimize your steps and let the belt do most of the carrying. You do not want to be caught at the end of your work area sorting through boxes all morning or you'll never get caught up.
Stack is whack. It makes a mess of your work area and increases your chances of having misloads. Scan them, put them away, get back out for the next one. Now there are times where there's too much coming at once and you have to stack it so you don't miss anything, but just remember that they won't load themselves. Also carry as many packages as you can into the truck at once as you safely can. If something is too big, stick it under the belt and save it for the end. If you need help lifting something, it's better to ask than to risk hurting yourself. Walk at a brisk pace, never run.
Shipping labels should face to the front, upwards, or outwards. Keep all bulk together as best you can.
Finally, don't let supervisors intimidate you. I know at least in my building a couple of them are only supervisors because they couldn't load worth a damn themselves. Bust your ass for your 30 days, work through your break if you have to, show them you care and don't mouth off. Once you gain seniority, never skip your break, work at a safe pace, and if your supervisors are full of

, be sure to let them know.