They say humidity should be high. To achieve this you can either cover the plants with a plastic dome or create a dome with cling wrap. However, I'll pass on something that I learned today that may or may not be true. I have a 50% success rate with clones and I thought it was because of lack of proper humidity. Covering didn't much help. I talked to my local hydro clerk and he thinks my problems are because of initial over watering. I clone in coco but where I think I have been going wrong is saturating the medium after inserting the clone. Coco is supposed to drain excess water, but apparently that's still too much water for a clone. According to this guy, domes aren't really needed. All that's needed is to make sure the initial watering is pretty light (no matter what medium your using) and there after light watering until the clone develops roots. Once it develops a root or two then it should have no problems standing tall.