Grew up on the farm backing all sorts of things. The longer the tow vehicle and/or the shorter the trailer the harder it is. I once backed my 4x8 trailer from the street into the ally with my 29 ft coach on the first try. BUT it did take many small corrections including short (1-3 ft) pull forwards to keep control of every thing. I am not going to try it again and ruin my record!!
The key as mentioned above is very slow and early small corrections. Be quick to stop and pull forward a foot or so to reduce the angle as needed then back slowly again. Also, either use your mirrors or look over your shoulder but do not try both ways during the same backing operation. If you do, you will turn the steering wheel the wrong way every time. I like backing long things!