really good tips from warps, I'd add a few other things.
Keep the bed as low as possible. No matter what other tricks come into it, a low bed makes every car easier to load. We used small diameter wheels and tyres, and low axles, to keep the bed down. The higher you are the harder it is to load low cars, and the harder it is when you inevitably end up pushing a car on one day.
Design a good mounting system in. The best way is chain links at the front and straps through hooks at the rear....that will ensure it never comes off but allows you to tighten it down nicely. There are some good over wheel tie down options too. But keep in mind not every car will be able to roll when it goes on (eg after you roll it 7 times off a cliff, or after a wheel is torn off. of if it is a shell on the way to sims metal)
Put a cheapy super cheap winch on it. They live in the sun and die regularly so don't bother spending on something quality.
Think about onboard storage. Spare wheel (or even better 2), tiedown, basic tools, hand cleaner and rag storage (invaluable), jerry cans, spare tyre rack. On the other hand, don't put too much general storage in because a trailer is almost always not secure (eg don't store expensive tools on board). Also depending how serious the events are, consider how you might secure the jerry can filler necks from silly buggers.
These days, wire in a reversing camera on the rear of the trailer as well as the car to make life easy
Get a big jockey wheel.
Make sure it is well painted and that the wiring is a quality install and well protected.
I've also lived with a cheapy that was fixed up over the years, it ended up working OK in the end but it was called the death trailer for a reason for a lot of it's life. In the end it took 2 new axles and suspension to get it to tow right, we spent twice the up front cost fixing and maintaining it. I'm buying new next time.
And finally....I agree....don't lend it. People abuse stuff that is not theirs, the more you think you can trust them the worse it is. I lent mine to the owner of a workshop in seven hills (no cost), it came back with a tyre that didn't hold air, gouged rim and bent axle, but "no way, nothing happened".