Take a coffee can and fill it with about 1/3rd sand. Get a bat with some sort of foam or a bat with something that will offer friction like surf board pads. Place the coffee can on the bat, start the wheel slowly, and start tapping the can with your left hand (if you are right handed) until it goes to center. Then place it off center and start tapping again until it gets to center. At no time are you to try to understand with your mind where to tap the can. You just tap it and when it gets to center you move it off and start again. Do this for 5 or 10 minutes and then stop. Do this every day for 3 weeks and you will be able to do it like a pro.
The reason you want to use your left hand is that you will have a trimming tool in your right hand and you don't want to have to put it down.
Tapping to center is way more flexible than using say a Giffen Grip. A Giffen Grip is a very useful tool and I would not want to be without it -- I use it to trim mugs when the bottoms are too soft to hold at center after tapping, but tapping to center is so versatile and quick that it is the best solution the vast majority of the time. It will save hours in the course of a year for almost anyone who makes pots.
Say you have a pot which needs to be trimmed in a chuck. (I sign my chucks "Charles" because they are very formal). You tap the chuck to center and then place the pot in the chuck and make sure the bottom of the pot is horizontal and then you have to center again because the chuck is not quite round and neither is the pot. So you tap the chuck to center again. It's not the chuck that you center but the pot.
Now you can trim the pot and if something goes off center you can just tap it back.