So, I've titled this post overly ambitiously on purpose. Having worked on a couple of projects, this is really just going to be a matter of tips and hints for the ne'er do well who aims to follow.
1) Have a story. There is nothing more paralyzing than getting halfway through a script and realizing that you don't know what comes next. Trust me, I do it all the time and it rarely goes well.
I'm a rebel when it comes to outlines, I hate the little creeps, but some amount of outlining is necessary. Comics, like music and comedy, have an element of dynamic timing. Don't believe me? Go back and look at some of your favorite books and see what the writers and artists do. The big daddy of them all is the flip page reveal:
Character 1 says, "And it'll take hell to stop me". Someone off panel says, "Why don't you try me first." In the next panel, character 1 sees who it is and shouts "You! You were supposed to be dead!" End of page.
This is a time honored tradition both in comics and television. While tv has the option of revealing before or after the break, comics have to make you pull the next page. BOOM! ADD! Or reveal, you know, one of those. Real experts at this are Brian K Vaughn, Brian Michael Bendis, and Joss Whedon.
2) Setting - this is one of the most commonly overlooked pieces to the comic puzzle. So many writers take it for granted. The first thing you should remember is that your artist doesn't have that luxury. More often than not, things happen somewhere. If you don't say where it is, they have to assume. Some big books can get away with this. For example: a ridiculous number of things in Marvel happen in either Times Square or Central Park. Real New York writers and artists feel more genuine. They know where things happen. When Peter Parker has to go from Flat Iron to Soho, they know how long that'll take.
If you're good and careful enough, your setting will reward you by being a character all its own. For an example of this try either DMZ or the classic The Sandman. DMZ takes on the real New York as an urban battleground. Where things are is of the utmost importance. One could argue that in this book Brian Wood's main character isn't fly by the seat of his pants reporter Matty at all, but NYC herself. In the case of Gaiman's "The Sandman" I never cease to be amazed. Gaiman has fashioned a world so complete and full that it becomes an exterior extension of a hopelessly inward looking character.
3) Get to know your artist: Every comic is different, some stories are different, and artists (as a whole) are different. Beyond that, artists are also different from one another. You need to know where your artist's strengths lie. You can't write the same thing for every artist. A lot of layout and panel structure will be up to your artist, but be sure to give them something they can work with. Some artists live for the splash pages. Some live for details. Some just like a challenge. Let's take two great and well known artists: Jim Lee and JH Williams III.
Jim draws in mostly blocky boxes. He does exaggerated figure and heavy crosshatching. Jim is ideal for drawing the dark, the gritty, and the violent. Jim's look dominated comics for a while, but he has nothing on JH for what JH does best.
JH Williams III is currently working on Detective Comics and has a huge amount of buzz around himself and Batwoman. Williams makes beautiful superheroines and painfully normal real people. He is also a master of page composition. You will rarely see just one picture on a page from Williams, even if it is a two page splash. His panels will be strangely shaped, his characters reaching from one panel to the next. While Jim is a master of the image, JH is a master of the page. You may not see as many giant posters with Williams' art on it, but open to any two pages to Detective Comics and you'll feel the electricity.
If all else fails, ask your artist what they want to do. If they read your scripts (assuming they like them) there are bound to be a few things they'll be excited about drawing. Figure out what they need and give it to them. Check out Jeph Loeb's stuff. He has a larger hater base than almost any writer I've seen, but he's still in business because he gets the best artists and lets them do what they do. It is a team sport after all folks.