Oh boy. That's a tough question.
Unfortunately there's no exact science behind this and it sometime it's just plain luck.
Mass appeal.
If you want readers, your art needs to have mass appeal, whether to the general public or a specific group. If your art doesn't draw people in, they won't hang around to read your comic.
Get noticed.
This one is out of your control. You can advertise and market your comic, but if you don't catch the eye of the right people, it will not help.
However, you maximise your chances of getting noticed if you become active in forums and such.
Consistency.
It is extremely important that people know that if they come back next week, they'll see a new comic. There's no bigger turn-off than trying to follow something with sporadic updates.
Also, keep the art and theme consistent. Don't suddenly jump the shark and change the comic's direction.
Stick to it.
This one is hard. Sometimes you just have to stick to it to make it work.
I'm the worst at this (there are even a few entries in TV Tropes where I'm placed as an example of someone that can't stick to the one comic), but try not to despair and give up too quickly before giving people the chance to warm up to your work.