I have a ProTank 2 that I've re-wicked the head of with the stock coil. I got a bag of organic cotton balls from Whole Foods (WalGreens I think also carries them) to use as the wick and it seems to be performing well with no leaking to speak of.
Basically, I soaked the head for a bit to loosen up the gunk on the coil, popped the chimney off, pulled the flavor and coil wick out. Dry burned the coil (use short bursts, brush ash off coil, rinse, repeat as necessary) then threaded some of the cotton through the coil. I don't twist the cotton before I pull it through so it has better saturation (learned that the hard way).
Threading is a bit tricky, because it will kind of collapse the coil in the direction you're threading, but you can fix the coil once you're satisfied with the wick with some really fine tip tweezers. At this point you can play with the spacing on your coil a bit to space the winding out a bit more evenly. I don't know that it actually makes a real difference, but in my head it does.
Once you're threaded, cut the wick to size, then cut a small piece for flavor wick. At this point I dry assemble and do a few test pulls (attached to the battery, but don't actually hit the battery button or you'll burn the cotton and have to start over) to make sure airflow is okay, knowing that it will change a bit when the wick is saturated.
Once you're satisfied with the build, reassemble and fill. If you notice any leaking, try adding a little more flavor wick or try a bit more cotton in the coil wick. It was pretty trial and error for me to find the right amount to use for it to perform the way I wanted.
Anyway, sorry for the book of a post, but I hope it helps in some way.