Per the US Airways website:
http://www.usairways.com/en-US/dividendmiles/preferred/upgrades.html

Upgrade notification
We send you an email the first time we try for an upgrade – no need to call. We'll continue to check for upgrades throughout the day and send follow up emails if we change your upgrade status – right up until 24 hours before your flight. If we're unable to upgrade your reservation up to 24 hours prior to departure, we'll process your upgrades when you check in online or at the airport (if they become available).

To be placed on the standby list choose the 'Add to standby list' link when you check in online. You'll only see the option to add yourself to the standby list if all passengers in your reservation are Preferred. If you're traveling with a companion, request that you and your companion be placed on the standby list at the airport.

Please check your Dividend Miles account to make sure we have your current email address.

Preferred upgrade priority – advance travel dates
When you purchase a full fare (Y or B class) economy ticket, you are eligible for an instant upgrade to First Class. After your reservation is ticketed, we will automatically attempt to upgrade you around the clock until a seat becomes available. We prioritize full fare economy tickets over all other fares.

If you book a discount fare (any fare other than Y or B class), we prioritize your upgrade by Preferred level and within each Preferred level, by the number of Preferred-qualifying miles you've flown on US Airways and US Airways Express operated flights in the last 12 months. We begin processing upgrades for all discount fares at either 7, 4, 3 or 2 days prior to departure, depending on Preferred level.

If we are unable to confirm an upgrade during our automated process, we'll try again when you check in for your flight. If seats are not available when you check in online, you can choose to be placed on the standby list.

To be placed on the standby list select the 'Add to standby list' link when checking in online. All passengers in your reservation must have Preferred status to have this option displayed at check in. If you are traveling with a companion, request that you and your companion be placed on the standby list at the airport.


Preferred upgrade priority – day of departure
•If we're unable to confirm an upgrade seat for you prior to departure, you may request an upgrade when you check in.
•Once check-in for a flight has begun, priority will be based upon Preferred level and then upon earliest check-in.
•Please advise the gate agent at the airport if you're traveling with a companion.


Companion upgrades
One non-Preferred traveling companion can upgrade when traveling on the same reservation as a Preferred member with a qualifying paid ticket. The companion will be upgraded at the same time as the Preferred member.


more info to add? PM aztimm.