I'm not so bad with black eyeliner on the water line, but I always make sure I have mascara on with it, even if it's just at the base. Probably just a habit, but I like to think that the mascara helps darken it up, therefore it's not so noticeable if it does start coming off.
Also I found that lining the upper waterline (verrrry tickly) not only darkens the upper lashline, but will eventually subside leaving a trace on the lower waterline. Hurrah!
Anyone have any tips for white eyeliner? I find it shifts very easily and ends up looking quite dirty eventually. It often clogs on my lashes as well.