Keeping item in list items "highlighted" on postback?

I entered all of the items in he asp list item fields, and set the first one Selected="true", however, when I click on one of the other items in the list and the page posts back the "highlighting" of the list item defaults back to the first one. How do I get it so that on initial start up the first item is highlighted, but whatever item I click on after that remains highlighted when the page posts back?? Thanks! 
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