Watch the ball. Especially on the serve/overhead. Don't let your head drop when the swing comes through, leave it up there. You have more time than you think.
This picture is taken with an exposed camera lens. Everything is a blurr except his head has remained exactly still. Myiesha said keep your eye on the open court and that is just wrong. I tell alll my players to watch where the ball WAS not where it is going.
steady head, eyes on the contact point longer... and if you can't see or imagine your shot hitting your target prior to contact then i guess it's a good idea to quit tennis and take up golf. :-)
Great pic of Fed. He was the poster boy for this tip. I think Nole is doing a much better job of keeping his head still now. Especially the forehand side. Take a look at his earlier years.