You don't have to do anything--as long as your WP number is in the reservation, and it reflects your status. Others have reported that they needed to manually remove and reenter their WP numbers in reservations they made before achieving elite status to get the status to show for those reservations. But once your number and status are in the reservation, it's up to fate, luck and NWs computer. There's a pecking order, and there are a few routes and flights on which even some Platinum Elites end up in the back (hubs to LAX and DCA seem to be a problem at peek times, for instance). But there are plenty on which Silvers have a great chance for an upgrade.

Good luck.