Aside from using points to upgrade in advance (when upgradable seats are available), the only ways to do this are:
1.) Change your ticket to a true First class fare
2.) Hope there are seats still open on day of flight and purchase the upgrade during the online check-in process(will cost about $250 USD each way)
The coupon connection is not a bad suggestion. Basically, if you find "low" first class award availability on us airways.com(which is difficult in the first place), a US member with points can request an upgrade for you. In exchange, you would offer a trade for points you do have.