Establish some sort of storyline and basic script for your comic beforehand. Also, don't work a page at a time.
I kind of disagree with the popular "just start" ideology of webcomic making... I understand that for some people, if they don't jump in they'll never start out of fear/perfectionism, but you should have some idea what you're doing before dedicating so much time/effort to your project, especially if you're dealing with a long-form comic/graphic novel.
One of the reasons my (long-form) comic's been in development limbo for so long is because I attempted to create a story out of virtually nothing, just a few notes and whatever ideas came to me the night before. There was no real foundation... until I wrote a script and outlined the story arc. NOTE: Don't get stuck scripting/outlining to the point where it's keeping you from drawing the pages. I recommend outlining the general plot of your story, including elements you really want to use/happen in the story, and then break that down into chapters, and then script the chapters, 1.5-2 chapters ahead so you can focus your drawings a bit more. YOU NEED A BUFFER. edit: Have between 8-20 pages already done or in the process of being done, depending on your weekly update schedule. (You might be able to squeak by with a smaller buffer if you only update once a week. As a rule of thumb: the more often you update your comic in a week, the bigger your buffer should be.)
Also, try not to draw a page at a time, at least in the very beginning, because if you suddenly change your mind about a certain story element, script, character, or overall composition of the page, you can't unless you start over, and it's very easy to get stuck in that loop once you allow it to become part of the process.
Best advice I've gotten recently was from reading @LordVincent's thread on creating a large volume of comic pages in a short amount of time: