Sign in on the auction house mule in World of Warcraft, check mail, and then scan the AH. Log in to EVE Online, pull up the markets, check what's up in Jita. Figure out if you need to make something to sell or use then log in the alts to do the dirty work. For some people, an alt is as simple as having a bank character that you send everything to for selling, or as involved as having a string of alts who can do every single production/tradeskill in your favorite MMO. Personally, Massively is one of those folks who has a bank alt on pretty well every MMO account possible and a couple of profession alts purely for the professions they think are fun or interesting.