Controlling focus on form with AutoPostBack TextBoxes

I have a form with several textboxes. All of these have AutoPostBack="True".  
My problem is that the focus disappears to goodness knows where after each postback. The behaviour I would like to enforce is as follows,
If the user hits the Enter or Tab key after filling in a textbox then the focus should go to the next field on the form (to be explicitly specified in my code). If the user clicks on textbox X after filling in textbox A then the focus should go to textbox X after postback, even if it is not the next in the list. In other words, I want the form to behave in the way that most users would intuitively expect.
I have Googled for potential solutions to the loss of focus after postback but the few that I have found were many years old and I am unsure that the advice is still relevant for ASP.NET 4.0. Some of these old solutions also appeared to be invalid for browsers other ...