Some of these i've just learned over the years. And some i have found on the internet while gazing.
Ever pull out of somewhere and BAM... License plate breaks off or gets bent to hell? Well the next time you mount it. Find some Stainless steel door hinges. Let that baby swing.
Trailer spings squeek? Get some white lithium grease in a spray can and while the boat is off the trailer at the dock. Coat all the spring pivots with that grease.
Safty chains drag? On my car trailer the safty chains always drag. So i took a bungey cord and tied it in the middle to the trailer. And fixed one hook to one chain and the other to the other chain. Will keep them off the road and still allow them to go where they need to while turning.
Do you have carpet bunks? Spray that carpet down with silicone spray. And that baby will slide and glide.
Affraid of trailer theft? Get a old worn out ball.. cut the threads off. And put it in the hitch and lock it in.