I have had similar issues but I have found causes/solutions for each
1. Tanks leaking - I have found this to be related not to the tank but rather to juice in the atty which is spread along the sides during the process of putting the tank back into the atty. Some of the tanks have a tighter fit than others, and if the tank is lose I just throw it away as they are cheap. An alternate fix is one layer of teflon tape around the tank if the fit is really loose this should stop or at least minimize the leaking. The only down side is each time you refill, you need more tape.
2. Juice in the mouthpiece - Sitting here looking at my two clear tanks I can see just a tiny bit of juice in there. If I pop the mouthpiece out of the top of the tank and rinse it out, making sure I allow it to dry afterwards the juice is gone, however I don't ever recall the juice actually making it to my mouth at all.
3. Mouth Piece popping out - If you bit the tank about half an inch from the top, the mouthpiece will pop out every time. You can see the horizontal line on the tank itself which is where it takes the least pressure to pop it out from. I haven't had it happen but my girlfriend got a surprise the other night in the hot tub when she was using "my precious" which is what she refers to my eGo as.
I carry mine in my pocket, lay it down, and all the other things you should probably not do and I have only noticed a leak one time, and that was a very small amount. Overall I don't think it leaks any more than my normal carts.