My guess is that you 'over correct' when backing.. Meaning, once the trailer starts to turn, you are late with following it in and have to over correct to get the trailer to go the other way..
What I do is once I get the trailer going the way I want it, I turn the truck tires back so I can start 'following' the trailer on it's path. If I need to get more turn out of the trailer, I adjust, but try to get back to 'following' the trailer as soon as I can.
Also, watching where the trailer tires are going is another way to help with backing. When you can't see where the trailer tires are, that's when a spotter helps, but the spotter needs to know how to backup too, so my spotter is only really there to say "STOP" if I'm about to hit something I can't see.. ;)
Like has been said, it's all about practice and time behind the wheel... I started with my tricycle 'towing' a hand truck and backing it into my camping spots when I was 5 years old! :)
Been backing up things ever since then and it's just second nature, but I still can't backup to this day if I can't see where I'm backing up to and have clear views out of my mirrors or looking out the window... ;)
Good luck!
Mitch