Lights and Electrical
The two most common ways to blow out a bulb are by immersing a hot bulb in cold water or by letting corrosion cause a short. Unlike hubs, some trailer lights can be removed before launching. Don't let the connector plug to the car touch the water.
Use only tinned stranded wire, which is more flexible and chafe resistant. Secure exposed wire every 18" to prevent chafing. Inspect the entire system twice a year for bare or chafed wire, and give all of the contacts a protective dab of grease. Don't forget the spare bulbs and fuses.
Laws governing trailers vary from state to state and are also subject to federal regulations. Further, they are always subject to change. Keep up with regard to your jurisdiction and rig.
When trailer lights begin to fail, consider total system replacement. Just cut wires at the rear end of the trailer and use old wires to pull new system wires through trailer frame. Simply hook up new light kit, and you have a brand new system.
Consider adding a heavy-duty flasher to your tow vehicle to help avoid the strain on turn signals (some turn signals will slow down to a crawl after a tow vehicle has been hooked up.)
Spare bulbs should always be carried for both towing safety and to avoid the day-ruining ticket you may get for towing without proper lighting.
Security and Locks
There is a wide variety of locks and security plates for trailers. Nothing is totally theft-proof, and many thefts occur when the thief simply removes the coupler latch bolt and tows the trailer away without a latch attached.
If you are putting your boat away for any length of time, you may want to consider removing one or all of the wheels from your trailer — this will make it very hard to steal and also extend the life of your trailer tire since it will not be exposed to the elements.
To protect your transom and outboard from the rigors and stress of road travel, consider a motor support. Adjustable struts hold your engine's lower unit in position, while built-in shock-absorbers prevent damage. Many manufacturers recommend towing with the motor down, but with many rigs this can lead to ruining the skeg or much worse when the motor hits the road if the transom or trailer isn't high enough. Figure out what works for your rig. Usually a motor support will solve this problem.