Its easy going forward as the trailer just follows along. But when trying to go backwards, the trailer seems to take on a mind of its own.
The first step for anyone learning to back a trailer is to learn to back in a straight line. When you go forward, you steer by moving the top of the steering wheel towards the direction you want to go. When you backwards, you steer by moving the bottom of the steering wheel towards the direction you want to go.
Practice backing in a straight line at a time and place where you will have no obstructions, people, or traffic to worry about. Move very slowly.
You should always have a spotter on hand when backing. The spotter's job is to warn off anyone from getting in the way, make sure your path is clear, provide warnings of potential hazards (not only on the road but also overhead), and provide some guidance about where to move the vehicle for proper positioning.
Have your spotter positioned well to the rear so you can see each other in the rear view mirror on the driver's side. The mirror will be very small to the spotter but it is still possible for the spotter to judge when he or she is visible to the driver by keeping an eye on the driver's side mirror. It is the spotter's responsibility to maintain a position visible to the driver. The driver should concentrate on the mirrors and on directing the rig - not on directing spotters.
There may also be other spotters to assist in backing but these should all work with the driver through the primary spotter in the driver's side mirror.
In those cases where a spotter is not available, the driver has to assume both duties. This means that the driver needs to stop the vehicle, set the brakes, and then get out and look around to see what might be in the way and to plan the attack. When space gets tight and crowded and the need for maneuvering high, getting out for a walk around to site the situation can also help relieve stress.
Video cameras and sonar equipment are getting sufficiently inexpensive to be considered as drivers aids. They can indeed be useful aids to provide warnings about when things are getting too close or to be able to see in places not otherwise visible. They do not remove the responsibility from the driver and the driver should be very careful to use them properly.
Video displays from cameras do not provide the driver a good reference and may display an image that is at odds with the driver's point of view compared to direction of travel. Use this technology like secondary mirrors. Do not use them to replace proper spotting or for primary driving guidance.
The key to why a trailer can seem to have a mind of its own when backing is the location of the pivot point (see the article about trailer sway for more on hitches and pivot points). In backing, you want to be on the long end of the lever so that it takes a lot of distance on your end for a small distance on the other side of the pivot. This is why it is easier to back longer trailers using a vehicle with a short wheelbase.
When you back a trailer, you are doing two things. The first is to move the pivot point to align the trailer to the direction you want it to travel. Second is pushing the trailer back along the path you want it to go. It is almost impossible to do these one at a time without special tow vehicles so you end up trying to do both at the same time. This can be a bit tricky. That path you take will not be the most direct but will require planning the track back as you move the pivot point towards the desired orientation.
When backing in a tight turn, the driver must be careful not to bind the hitch. This happens when the angle between the trailer and the tow vehicle becomes so acute that they touch at some point other than at the hitch itself. A rear mounted spare tire may start to push on a propane bottle. The bumper may push on sway control devices.
The forces involved in pushing a trailer around can be quite large. A turn so tight as to have the tow vehicle and trailer touching at other than the hitch point can easily create damage to the front of the trailer or the back of the tow vehicle or anything in between.
With the old teardrop trailers of the fifties, you could put the kids in the back bed (big kids might even cause the tongue to lift off the ground!) and a wheel (that came with the trailer) on the hitch jack post and manually pull the trailer around to get it into tight spots. This wheel essentially made for a tow vehicle with a zero length wheelbase.
A short wheelbase vehicle as, for instance, my 1973 GMC Jimmy, could pivot a 24' Holiday Rambler around the pair of wheels on one side without binding the hitch. A Suburban, with its longer wheelbase, could not do this and made getting into real tight spots a bit more difficult.
Longer trailers are easier to back because they respond to changes in orientation slower. Moving the hitch of a short trailer sideways will change its orientation much more than moving the hitch of a longer trailer sideways the same distance. When more change is required, you have more opportunity to see what is happening and make corrections.