Each one and one of the tips that are mentioned above are important and are not sorted. So please read carefully till the end before towing a trailer!
Check the tires: Since the trailer is not being in move daily, and not even monthly. We use it a couple of times a year. Since most of us store them outside, the tires may get damaged. The main reason for the damage is the weather conditions that may have a harmful effect. It may look like the tires still have plenty of tire tread, but the aging made it damage. Add air if needed according to the weight and the manual. Also, make sure that your tow vehicle tires are with enough air according to the towing manual. Make sure you have at least one spare for each.
Check all the safety attachment equipment. Again, since you don't use it frequently, make sure each item is in good condition and lubricated: safety chains, hitch balls, ball mounts, wiring (especially for the breaks), bearings, races.
Vehicle Maintenance: Before heading on a long trip, make sure to service your vehicle, and avoid stressful situations of being stranded on the side of the road: engine coolant, oil, brake pads, etc. Tell your mechanic, that you are planning to tow a trailer, he will know exactly where to put his extra attention.
For an easy tow process and avoid swaying, make sure you load the trailer correctly.
If you are planning a long towing trip (road trip, some kind of transportation), make sure you update your insurance provider about it, yes it may cost extra, but everything will be covered. Also, roadside assistance services may be included for your vehicle and the trailer.
When towing any size and type of a trailer, you are using more fuel. So grab few extra gallons and keep them for an emergency.
KEEP EXTRA DISTANCE FROM THE CAR IN FRONT OF YOU!
Be extra careful when parking the car.
Be extra cautious of potholes and wide turns
Take wide turns