DC Universe Online — Reputation & Elite Gear Reference
You can use a Renown Booster box to increase the renown you get from consuming a Commendation by 25%. So something 100 would give you 125 instead. A 400 one becomes 500. You don't need to have this active while running content. Collect your Commendations all day, then activate the Renown Booster Box, and consume them all at once. It works during consumption, so you only need it active for a minute or two while you consume them. If you don't have any, you can get them from Dr. Fate.