From what I learned from my driver.
PRE-LOAD:
Put big and/or heavy stuff on floor and out of the aisle as much as possible, labels facing out or forward with packages propped up on the shelf lip. Concentrate on avoiding misloads by going at a steady pace, your speed and accuracy will increase with more experience. Check that your PAL and shipping labels match and that you write sequence number on every couple packages
LOADING:
Start by placing all the heavy and bulky stuff at the bottom of your walls and progressively get lighter towards the top. Fill gaps in your walls with small packages. Use your leg muscles to help move big or heavy packages and bend at the knees instead bending your back to lift things. Avoid getting hurt and work at a steady pace.
UNLOAD:
Work at a steady pace and put packages 1 at a time on your conveyor or roller. Put the PAL labels on a surface that will be visible when a pre-loader loads onto the package car.