For cuttings in 4" pots I put a gallon sized freezer bag over the cuttings and pot. It is open at the bottom to allow drainage but helps keep the humidity high at the cutting. I'll remove and water every few days. Anything can be used. If the pot is larger you could make a wire frame and cover it with clear plastic wrap from the kitchen or a dry cleaner bag.
Do a google search for propagation supplies and you will find flats with clear plastic covers. These also work if the cuttings are short, these flats are usually sold for starting seeds.
Larger commercial operations use a greenhouse and misters to keep the humidity high, the entire greenhouse is a large tent. For more information check Brent's articles on cuttings at Evergreen Gardenworks here: http://www.evergreengardenworks.com/
I've used one gallon water and soda containers with the bottoms cut off and place those over one gallon nursery pots filled with perlite this season with really good results. With the cap left on as a vent you can control the amount of humidity inside the system simply by loosening it and remove it all together once the cuttings harden off and start to root.