This video is my favorite, because it contains some of the best tips I've accumulated from my time writing comics. A good many of these came from my editor at Dark Horse, Mr. Dave Marshall, to whom I'm eternally indebted. Let's get into it, and I'll throw some bonus tips in as well!
Don't keep secrets in your scripts
You may have a really cool twist that you want to preserve for the end of your book, where the villain is really the main character's mother in disguise. But if you don't tell your artist that the first time the villain appears, they may just draw the villain two feet shorter than the mother! You need to communicate everything your artist needs to know in your panel descriptions, not just what they need to know in that panel. So as I say in the video, if the character is wearing a necklace that needs to start glowing on page 50, mention it the first time they appear wearing it.
Speaking of speaking order
This is one of my favorites, because it's a little big like magic. You generally want the character who speaks first in a panel to appear on the left, otherwise you end up with a bunch of long, gnarled tails to your word balloons. One way to ensure this is to mention the character who speaks first in the panel first in your panel descriptions. Artists tend to subconsciously draw the characters who are mentioned first on the left. You could always specify the order in your script as well, but this is a good way to layer in the info easily!
Don't overdo the dialogue
If your art can tell the story without any dialogue, you're doing a good job of visual storytelling. So don't go and drown your art in tons and tons of word balloons. My tip for this is to find a comic you like and find a slightly long-looking word balloon, then type out the contents and see how many words it is. It's probably fewer than you think. Balloons fill up fast!
Instead of overstuffing your balloons, try breaking up a long sentence into two different balloons. You can try this using an ellipsis (...) or dashes. (Note: in comics, it's always two dashes - - never one!)
These are used for different things, though. Ellipsis is used for a character trailing off, which makes it good for breaking up balloons. Just don't forget to put an ellipsis at the start of the new balloon, too! Dashes are used for when a character gets cut off or interrupted.
Speaking of letting your artwork breathe, try mixing in silent panels as well! These make sure you're letting your art help tell the story!
Here's a bonus tip I couldn't fit in the video. Take a look at a comic — you may see that some words get special emphasis. This is accomplished through bold italics, which help the reader understand which words are receiving emphasis, and bring some life to how they're read. If you want your dialogue to have bold italics, include it in the script so the letterer knows which words to emphasize. (Note: not all comics have these anymore. It's a matter of personal preference.)
Speaking of personal preference, a lot of comics have captions (little boxes, usually in the top corner) that can be used to provide narration, show what a character is thinking, or just provide a transition ("Meanwhile..."). You don't have to use captions, but they've generally come to replace thought balloons, which used to be used to show what a character is thinking. You'll still see thought balloons occasionally, but usually in self-consciously retro comics, or in comics for younger readers.
Even Out the Odds
Comics are often physical objects, which means that the odd pages are always on the right-hand side of a spread (starting with page 1, of course) and evens on the left.
It’s a good idea to make sure any major surprises happen on an even page (so they’re not spoiled by someone seeing them on the spread), and to try to have moments of suspense happen at the end of odd pages. Or, even better, at the end of every page, to create a sense of momentum.
Whatever you do, don’t ask for a spread to start on an odd page (i.e. pages 33-34). It’s physically impossible!
I hope those tips were helpful. If you have any questions, I'm happy to address them on Twitter. And thanks again to Great Dog for the chance to share these!