Take the batteries out of the controller and replace them - often cause of poor synch.
Retry synch my holding down button - should spin the lights then synch a bit quicker.
I use good quality rechargeables in the controllers.
Cheap batteries dont last.
From nothingleft2005:
There's a little button on the front of your 360 and the same button is on the front of the controller. You hit the button on the 360 and the green lights around the power button start to circle. Then you hit it on your controller and the green lights around the xbox guide button start to circle. In a second or two they'll find each other and both will flash at the same time, and it's synced up. I think there's a little symbol of air waves next to the button.