Contrary to some advice above, learn to back up properly before doing it exclusively with your mirrors. Look over your shoulder when backing just like they told you when you learned to drive. Using your mirrors before you intuitiely understand what happens when you turn the wheel this way or that way can be confusing.
the empty supermarkt practice tip is a good one. You have lots of room, and white lines to aim for, and hopefully few people to watch you make the inevitable fool of yourself (he-he).
A little trick that helps is to remember that the trailer will turn in the same direction as the bottom of the steering wheel. In other words, if you place one hand in the center of the wheel at the bottom and move your hand to the right, the trailer will turn right.
Also remember that few trailers will back perfectly straight very far, so do not worry if you wobble a bit over a longer distance. Just make small corrections as you go.
Lastly, keep in mind that if you get where you cannot turn it into the spot you are aiming for, you can always pull forward a bit to straighten, or even turn the trailer and take another shot at it. Sometimes you need to make 2 or 3 back/forward moves to get it right.
Most of all, just take your time.