m0use has a good suggestion - a/c the room the tent is in... I know several growers who do exactly that.
When I have MY a/c going, the humidity in my tent goes from 60 to 47% pretty quickly...
If that's not feasible for you, a cheap way of getting some of the water out of the air is to take 2 5-gallon buckets, drill some holes in the side and bottom of one of them (don't go higher than say 6 inches on the sides), put it inside the bucket without the holes and fill it up with rock salt... the rock salt will take the water out of the air and it will drain into the lower bucket which you will have to take out and empty as frequently as necessary .... of course, that means you've got to have room in your tent for at least one 5-gallon bucket but this WILL work to reduce the humidity.
Good luck!