3. Know your brakes
Once you've got your truck all loaded up and hitched, it'll be significantly heavier than the unladen truck. Some trucks accelerate and turn nearly the same when towing, but all will require longer stopping distances, particularly from higher speeds. Knowing the condition and capacity of your brakes is a key safety element, so be sure to inspect them (or have them inspected) before heading out with a trailer in tow.
Part of knowing your brakes is also knowing your route. Are you going to have any long hill descents on the way to your destination? If so, even new brakes may not be up to the task if the truck is loaded to maximum capacity--extended braking periods can generate excessive heat, causing the fluid in the braking system to boil and the brakes to fail. If possible, route around long descents, and if not possible, be sure to have your entire braking system evaluated regularly by a professional, including keeping to regular fluid change intervals.
4. Know your lights
If the drivers behind you can't see your taillights, and your trailer has no lights of its own, how will they know you're slowing, stopping or turning? Wiring harnesses or adapters to connect your trailer to your truck's lighting system are available from many aftermarket outlets, and are required equipment in most states.
It's not enough to just hook them up and forget about it, however--be sure to check your trailer's lights and signals before every outing, as bulbs can burn out and the wiring and connectors can fail due to exposure, friction, and vibration.
Trailer lighting is a fairly simple and straightforward subject, but getting it right will be much appreciated by the other cars on the road.
5. Know your skills
Practice, practice, practice! If you're new to towing and trailers, you probably aren't an ace at backing up and turning with a trailer attached, particularly a loaded one. Heavy trailers and uneven surfaces may change the low-speed throttle response of the truck when backing up, making it tough to start moving smoothly and slowly. Practicing in a safe environment will help build the feel necessary to get the trailer going safely.
There's also the skill of getting a trailer to go the direction you want it to--it's often counter-intuitive how a trailer will behave behind a truck, as it can move in the same or opposite direction of the truck's movement depending on the angle between the two. If you're having a really hard time getting it right, consider asking a friend or family member with experience to give you some in-truck pointers.
Practicing before heading out into the wild will also expose any weaknesses in your setup, whether it's the hitch, the trailer, the weight distribution or the mirrors. It's important to see where the trailer is when backing up, and many trucks' standard mirrors aren't large enough to see and see around larger trailers. If you're having visibility issues, you can add a small circular convex mirror to expand the short-range field of view, or it may be necessary to upgrade the mirrors entirely to larger units, particularly with tall or wide trailers.