Hook up and cross the safety chains:
If, for whatever reason, the trailer detaches from the tow rig, the only things keeping the trailer under control are the safety chains. It's not good enough to just hook the chains to the tow rig -- they need to be criss-crossed. Hook the right chain to the left latch-point on the truck and the left chain to the right latch-point. In theory, if the trailer were to uncouple, the crossed chains will cradle the coupler as it drops, rather than allow it to dig into the road and possibly send the trailer cartwheeling.