I always run my 16" E rated tires at the 80 lb max. Trailer/axles are below their 12,000 lb rating with around 10,000 on the axles + tongue weight. But my tires often go before being worn out. A friend who has tire store says that it's getting harder to find reliable tires anymore with the offshore manufacturing. They replace them after a few years, but that's not as easy for most of us.

* Age - I had a "brand new" trailer tire stored inside for about 6-7 years. When I finally used it, it peeled like a banana after a couple of thousand miles. It held air and I ran on the steel belts for a mile or two until I found a good spot to pull off.

* I'm lucky in one respect in that I live where there are a lot of trees and that keeps the trailer and, more importantly, the tires in the shade. Otherwise I would definitely keep them covered any way I could, even if it's with cardboard or plywood.
The downside to all the trees is the leaves, birds, dust, etc. on the trailer from them.

* Dual failures - When one tire goes, it can overload and blow the other one on the same side.

* Tire/weight ratings - I like to have stuff rated a good margin above what I actually need for extra insurance. Unfortunately, E-rated tires are expensive enough. Anything more costs quite a bit more.

* Make sure the weight is properly distributed inside the trailer and it is level so one axle/tires don't get overloaded that way.

* Wheel bearings - I saw an obvious difference in bearing life when I switched from good, conventional wheel bearing grease to synthetics. I use Amsoil Synthetic Racing Grease but they have a variety to choose from. I also check the bearing preload several times a year and retighten them if I feel any more than a faint looseness. But be careful not to over-grease. (Don't use a "heat" gun on your tires & hubs. Use a temperature gun. There are a lot to choose from - One of the cheaper temp. guns.)

* Brakes - I manually adjust the brakes several times a year. It seems that if clearances are allowed increase much, it's harder on them when they do engage.

* While we're at it, don't forget to check the lug nuts too! I found a brand new tire on a trailer wheel on the side of the road once. Wheel had its stud holes wallowed out!