the way I do it is to take a ten galon tank with a screen top and cover the top with seran wrap. whatever percent the humidity is you cover that much, say you need 75% humidity then you cover 75% of the top. works pretty good. just make sure you still have enough ventilation.
What type of T are you keeping? And also how large is it?
For my arboreals I use split glass aquariums and cover part of the top like johnny said. For my terestrials that need humidity, say they are getting close to a molt or something, I also cover the top of a kritter keeper with something to hold in the humidity.
For arboreals, you can also use a large jar and poke holes in the lid and sides, these hold humidity well also.
Many people use beanie baby containers - which would facilitate higher humidity. Just poke holes until you get the humidity and ventilation you need. Craft stores like "Michaels" here in California carries them.