The best way I know of to practice is to go to a large empty parking lot and ride towards a parking spot, on the line, then turn and come out three places over. Work at it both directions. When that becomes easy, do two and a half, then down to two if you can.
Please excuse the ASCII art, but this is the general concept:
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Go in on the line, turn before you get to the end of the parking spot, and come out on the other line. For a good U-turn, you need to be able to come back out two lines over instead of three.
Practice stringing together a bunch of left and right turns using the row on the other side and you should have it down.
Then you just need to remember to do it the same way when you need to do a U-turn somewhere.