As mentioned hanging food in a silnylon stuff sack by a rope to a branch is a good prevention method. However it can be time consuming to throw the rope and manage to hook it over the branch. Tying a stick around the rope helps give it enough weight to throw.
Other methods include not camping at camp grounds. (As possums will get to know the area has food and where to look)
Double bag your food in zip locks or odur proof bags. I use a zip lock then all my food goes in a waterproof siknylon rolltop sack. Then put them in your pack. Preferably the pack has a waterproof liner which is also rolled up and secured. All your cooking gear should be clear of food scraps and also inside the sealed liner. Make sure there is no food wrappers in hipbelt pockets or trousers pockets. Rubbish also double bagged and put in the pack liner. So three layers of smell protection and then the closed pack.
I mostly use the later method combined with keeping my pack in the tent. I also rarely camp at campgrounds. Haven't had a possum get into my stuff so far.. touch wood.
Final method.. move to NZ. They hunt em there..