I've used the eGo-C for several months now and have zero problems with leaking. Here are some tips -
1. Dragging too hard is probably the #1 reason why people have leaking problems with the eGo tank system. The eGo-T (and C) require a soft, slow drag, doing fast, hard drags like most people smoke analogs will pull more fluid through the wick than the atomizer coil can handle, resulting in the build-up of juice and eventual leaking through the bottom of the atty.
2. Make really sure that the tank is fully inserted, there shouldn't be any gap at all between the small shoulder on the tank and the top of the cone when you run the tip of your fingernail over the junction between the tank and cone. Usually there is a noticeable click sound and feel when the tank is fully seated but every once in a while I come across one that takes extra effort to get it inserted all of the way into the cone.
3. It may be that the wick is a bit too far down in the atty spike. Vicki at Cignot has a
short video with an excellent explanation and demonstration of how to fix that.
I hope this helps.