Distribute weight evenly to make driving easier. If you load all of the heavy items on one side of the truck, it could make it difficult to steer the truck. Aim to spread the weight of your belongings evenly throughout the truck.
Place the heaviest items against the walls. This will keep the middle of the space free for easier access and loading and unloading.

Place pictures and mirrors between mattresses or other soft items. You’ve already carefully packaged your pictures and mirrors, but it won’t hurt to give them an extra layer of protection. Slide pictures and mirrors between 2 mattresses to give them the greatest amount of padding.
If you don’t have more than 1 mattress, you could use the same trick with sofa cushions.

Load couches vertically. Couches and loveseats can take up a lot of precious floor space. A great way to avoid that is to place the couch on its end (arm side) and make it vertical. This is especially helpful if you were unable to take your couch apart.

Secure furniture with straps. Many rental moving trucks come with straps that you can use to tie down your belongings. When you pick up the truck, ask the agent if straps are included. Then ask for a brief demonstration of how to use them.