I can't remember where I saw this trick, but here's what I do. I picked up one of these (http://morebeer.com/view_product/18250/) from the local homebrew shop. It's a cap that has a ball-lock connection for a 2-liter soda bottle. When a keg runs dry, I fill the 2L bottle with Star-San or BLC, attach it to the gas line to pressurize to about 20psi, then FLIP THE BOTTLE UPSIDE DOWN. Opening the tap allows the pressurized cleaner to run through the tap and flush out the line. Works like a charm, plus I can carry a 2L bottle of delicious homebrew wherever I want without losing carbonation.