I'm certainly not an eGo-C expert as I just got mine yesterday morning but I did do a lot of reading about it before I bought it and so far I've gone through almost 3 tankfuls without even a hint of leaking. Anyway, here's my suggestions -
1. Pre-punching the tank cap doesn't seem to always be necessary (although it may help in some cases). I didn't pre-punch the cap and from what I've read, many other people who don't have leaking problems don't pre-punch either.
2. Do not fill the tank all of the way full. Instead, only fill the tank to where there's about 1/8 inch of air space left between the bottom of the cap and the top of the liquid. (It may help to hold the cap next to the tank to get an idea of where you should fill to in order to leave a bit of air space between the cap and the juice.)
3. Make sure the tank is very firmly pushed into the cone so that the tank cap is fully seated on the atomizer.
4. It's not necessary to twist the tank after you insert it in order to get it to start wicking. Even with 100% VG juice, I easily got mine to start wicking by (a) dripping a couple of drops of juice onto the tip of the atomizer spike before inserting the tank, and (b) then doing 3 or 4 dry drags (that is, without pushing the button to activate the atomizer) before starting to seriously vape. The suction created by dry dragging pulls the juice into the spike to help start the wicking process.
5. My sense is that it is important to use a fairly slow, somewhat gentle inhale because the suction created by a short, hard drag apparently can pull the soft edges of the cap away from the atomizer spike, thereby leading to leaking. Don't worry that a slow and easy inhale will result in poor vapor production - you will get tons of vapor with the right technique (see, e.g., this
short video.)
6. It's also possible that the wick is a bit too far down inside the atomizer spike, which can cause flooding and leaking. Vicki at Cignot has a
short video showing how to fix that.
7. You can also try the
soft silicon tank caps carried by Cignot and other vendors. (See also this
forum thread that talks about the silicone caps.)
I hope this helps a bit.