I second on the side view mirrors and using the "swoop" method and practice. I don't have someone else in the vehicle with me to help me back up, so I had to learn how to do it myself. I learned how to use the side mirrors extensively, I learned how to use the swoop to help line things up, and I learned that getting out once at the beginning to "check things out" helped a lot. It took multiple tries to get comfortable with it, but I got to the point that I could back my trailer into my narrow driveway (at a 90 degree angle to the road) in the dark with no street lights and not hit the fence, house, etc.
One thing to remember is that there are other things you can use as a guide than what seems obvious. If there is a tree 5 feet in front of the space, then you can use that to help determine if you're in position. Learn how far windows, awning supports, etc. are along the sides of the trailer. If the rear awning support is 2 feet from the end of the trailer and is in position next to the tree, then you are 3 feet past the front of the space. It's not a perfect science, but it does help.