United does not have automatic free upgrades for premium members unless you're flying on a full fare Y/B coach ticket. So, using either miles or 500 mile certificates are probably your only way of being upgraded at this point.

There have been lots of threads discussing this and a lot of people prefer United's way, especially those that do not have the top level 1K or Global Services status...because it means that even top level flyers will sometimes sit in coach, opening up seats for those with Premier & Premier Exec status who want to upgrade.

To use United's 500 mile electronic coupons, call reservations as soon as you book your ticket and put yourself on the upgrade list. Clearing will be automatic 72 hours in advance if there is room, otherwise you'll be waitlisted.