Both of my clear tanks were fine, but I did have some problems with the smoke one until I figured out that the top o ring wasn't making good contact with the top of the chimney. I wound up sanding both ends of the tube to shorten it a tad bit and allow the chimney to make better contact. A little something between the top cap and o ring would probably have worked as well.
The two normal things that I know about that will cause leaks is having short wicks in the chimney and/or filling from the top and allowing the air pressure to push the juice above the coil deck when screwing the top back on.
I always make sure my wick ends are long enough to circle the shaft inside the chimney like this:
http://www.e-cigarette-forum.com/fo...clone-aka-dream-bt804-tank-7.html#post8675891
If you fill from the top, make sure you keep an eye on the juice as you start to screw the lid back on, and when the juice starts to move down a little, flip the tank, wait for the juice to drain back out of the channels, then finish tightening the top.
http://www.e-cigarette-forum.com/fo...lone-aka-dream-bt804-tank-11.html#post8767241
http://www.e-cigarette-forum.com/fo...clone-aka-dream-bt804-tank-7.html#post8702219
I always plug the top cap and fill from the bottom.