My son gets croup every winter, it seems. The doctors told us to run a steamy shower and let him sit in the bathroom, then open a window and let him sit next to it to get the cool air. This worked most of the time. Sometimes though, it got so bad that we had to take him to the ER and run a nebulizer (sp?). Twice it turned into Pertusis, even though he's been vacinated.
In my experience, it goes fast, usually only lasting a day or two for us, but it's really uncomfortable. It seems to be worst at night. Cool liquids helped his throat, and sometimes he'd need a fever-reducer like Tylenol. If you use a humidifier, make sure you clean it well after every use so that bacteria doesn't build up in it. We ran into that problem a few times, too.