Front to rear, new on front. I want the newest tire on the front - more significant handling issues should the front tire blow/flat. I'm about to do this again relatively soon. After 3,000 miles I moved the front to the back/new on front, and its now 1,000 miles later. I use continental grand prix 4000s ii. The wear indicators are still visible, but since I've gotten my monies worth out of them may as well be a bit preventative and not wait to the last possible mile.
Just noticed this is the hybrid bike forum, and my bikes are road bikes. Tires and mileage may vary, but still the same concept, newest tire on the front - don't want any issues in the front.