I kept mine running all the time, and just "fed it" periodically to maintain the bio-colony, but many people just keep a couple of filters in the sump (sponge filters, bio-balls, etc) so they can set one up when they need it...A colonized sponge filter, and a tank of water from the main tank (presuming there are no parasite issues) and you have a functioning QT, and all that's needed is a heater, and some 3-4" PVC pieces for the fish to hide in/behind. Here's a pic of a simple QT with a home-made bio-ball filter:
IIRC, I stole that pic from Cheeks...
but it's a great example of how simple a QT can be...