Nothing is leak-proof.
Tanks are like owning Gremlins. They come with rules.
1) Don't over fill it.
2) Don't twist it on too tight.
3) Always check your o-rings. If one's lost, or the base o-rings are damaged, it will leak.
4) There may be a hair-line fracture in the tank that you cannot see. Is it leaking from the bottom metal base?
5) How old are the wicks? If the wicks are older than two weeks, it may just be time for a wick change.
(EDIT: To anyone reading, these rules apply to "top-coil" atomizers.)
(For bottom-coil clearomizers, the rules are a tad different.)
iClear16's produce great vapor and excellent flavor, but they are very fragile. They do not like pockets or purses. Any sort of lateral pressure from the side can cause them to break.
You may want to look into iClear 30's if you like the 16's. They will make your device top-heavy and you won't be able to use a lanyard, but they do stand up better than the 16's.
EDIT: Other than that, I agree with the above poster. If you have a B&M store with a knowledgeable staff, ask about cartomizer tanks.