Just wanted to chime in on backing a trailer:
Go SLOW. Very slow. If your truck has a low range, use it. That way you can see and correct unwanted behavior before it gets to the point where you have to go forward to correct mis-alignment.
When backing, try to visualize the arc(s) necessary to move the trailer into the desired position. Get out and look things over if necessary. Also look for fixed obstacles like trees or gate posts that might interfere with the process; it's way easy to bang the front of your tow vehicle into things while you are concentrating on what's happening behind you.
Once you have a clear picture in mind, use large input from the steering (all the way to the stop) to get the trailer pointed in the correct direction, then immediately straighten the steering to put the truck in line with the desired turning arc. From here, use only minor steering input to keep things in line. This is important enough to reiterate: big steering to align, small steering to maintain. Once this sinks in, backing will become much easier, and you will generally be able to speed things up a bit.
Silly as it sounds, practice in the barn by "driving" a small wagon backwards thru doorways and around feed bins, etc. It's the same geometry.
Also, assuming you have a tractor, get a "tow" attachment for the 3-point, and use that to move trailers around in The Yard at home. You can achieve much more precise positioning that way, and it's good practice for when ya gotta do it with a big truck/less visibility. Given the opportunity, I will use my tractor to get a trailer out from its parking spot, get it loaded, pull it out to the street, and then finally back the truck up to it, hook up, and drive away. The process reverses when I get it home.
One final trailer trick: I have an inexpensive wireless back-up camera. Rather than mount it on the license plate per usual, I attached a couple of strong magnets to it. When I need to connect a trailer, I position the camera to where I can clearly see the ball on the screen, and thus can generally back right up to the hitch in one go. "That's Cheating!" "Yup :-)"
Steve Jernigan KG0MB
University of Colorado