Let me say straight up that I don't believe it's possible to come up with a set of rules that are always applicable, for everyone, in every situation. Any halfway-decent cartoonist probably looks upon any set of rules as more of a set of challenges – "how many of these can I break and still make a readable comic?" – rather than the artistic equivalent of physical laws, without which nothing works at all. So I'm offering these "rules" just as a set of guidelines I find helpful. These are some things that work for me... except when they don't. I trust they'll be taken in that spirit.
1. Clarity is Your First Priority
Comics can do a lot of things - they can look beautiful, they can challenge you intellectually, they can show off your incredible range of drawing skills - but if you can't understand what's going on, all of that effort is for nothing. So I nearly always try to make the information that the panel or page has to convey as clear as possible – and as easy to follow from panel to panel as possible – before I even begin to think about any of that other stuff. Building your compositions according to classical principles is great if you can do that as well, but if you have to make a choice between beauty and clarity, or cleverness and clarity, or mood and clarity, clarity trumps everything. The only exception is if there's some narrative reason for making the information obscure.
I'm referring here to traditional narrative-based comics, of course; there are other kinds of comics (poetic/mood pieces, abstract comics) where none of this applies. (But if I qualify everything, they'll stop being rules, won't they?)
2. Treat the Lettering as a Part of the Artwork
Computer lettering is a great time-saver, but it's led to some really difficult-to-read comics. Sometimes that's because the lettering itself is done sloppily; there seems to be a school of thought that, because you're using a font, no further care or attention is required – as if perfectly-formed letters are the only difficult bit, and balloon shapes, dialogue placement and correct spelling and punctuation are things that will take care of themselves.
Other times the problem seems to be that the artist hasn't thought about the lettering at all, so scenes are staged in such a way that there isn't enough room for the text, or the dialogue is forced to flow in a counter-intuitive way in order to accommodate the existing pictures.
Me, I always place my balloons before I draw a single line. Sometimes I'll go so far as to completely letter the page first, before I pencil; other times, I'll just rough in where the balloons will go. But I always, always place the text before I do anything else. The balloons are an element of the artwork; they need to be a part of your compositions, and ideally they should come from the same hand as the hand that draws the pictures.
I have a personal preference for hand-lettering for a number of reasons, most of which boil down to "it looks better"; I will resort to digital lettering if limited time makes hand-lettering impossible, or if I'm specifically asked to do so (for ease of translation, for example; or company policy; or if what I'm drawing is supposed to fit in with other comics by other artists and the font is being used as a unifying element). But it's always my second choice. I have a few fonts based on my own lettering which I use when I have to. Generic comic book lettering fonts are to be avoided at all costs if you want your work to have any personality of its own – they should be strictly reserved for corporate superhero comics, which (these days) aren't supposed to have a personality.
I don't believe artists who say "I can't letter." I think this is hogwash, in every single case. If you can draw, you can letter – the same rules of visual balance, aesthetics etc. that apply to drawing also apply to calligraphy. Just study other letterers, copy their letter forms, and practice a bit. Spend the same amount of care lettering as you would drawing. You'll make better-looking comics at the end of it.
3. Show the Feet Once Per Page
This is a general rule of thumb I try to stick to, although it's occasionally not appropriate for certain kinds of comics (newspaper-style gag strips, for example). But it's a good idea to remember to show your characters full-figure now and again, particularly if you're doing action-light, dialogue-heavy scenes. The once-a-page rule keeps me aware of the issue.
4. Keep Up the Momentum!
Something I try to do is to avoid having characters standing in one place doing nothing for two or more panels in a row – unless there's a timing thing that's required, like for certain types of gags. The rest of the time, even if it's not called for in the script, I try to have my characters constantly doing things with their hands, or moving from one place to another – always, always moving forward in time. Even if you've been given a script where nothing is happening, you can make it feel as if something is happening by keeping up the momentum visually. If two characters are standing still and talking for six panels, you can have somebody doing something in the background so the scene doesn't feel frozen in time. Someone opening a jar of pickles. Anything. Just keep it moving.
5. Remind the Reader Where Things are Taking Place
I'm conscientious about backgrounds and environments. Not that I feel they need to be meticulously rendered in great detail in every panel; in fact, that's probably a drawback, distracting you from the characters the story is about more often than not. But I think it's important to clearly establish visually where things are taking place, and to keep reminding the reader with little shapes, silhouettes and details that keep the environment in mind. These don't have to be elaborate – just little cues are all that's required – but they ought to be there.
I generally allow myself a generic, non-specific background every other panel, tops – less than that if I can help it. Two background-less panels in a row is something I try to avoid. It's part of creating a credible, internally-consistent reality in your comics. Which brings me to...
6. "Realism" is Less Important than Internal Consistency
I think I'm probably in a minority on this, and a lot of it has to do with the stylistic path I've chosen to follow, but I'm much less interested in drawing a thing "well" than I am in making it credible in the stylistic context within which I'm working. And a big part of making things credible is consistency. In other words, you can set any arbitrary standards for "realism" in the world you've created, but once you've decided what they are, you need to apply them consistently in order for that world to remain believable.
7. Keep It Looking Pretty
It's become fashionable to say things like "comics are all about story" and while that side of things is obviously important, I think focusing on narrative at the expense of design and beauty can be a mistake. Ideally, you want your pages to be a treat for the eyes at first glance, because that is what's going to suck the reader in initially – an ugly comic is a much harder sell than an attractive one, no matter how good the story is. To that end, I try to break up my page designs with occasional tricks like circular panels, borderless panels or varying border widths (ideally at points of narrative emphasis rather than arbitrarily, but I'm not above an arbitrary use of these tricks once in a while). It takes surprisingly little effort to add a bit of graphic variety, and it makes the pages a lot more interesting to look at without sacrificing any narrative clarity. Well-placed areas of black and use of textures and patterns in your backgrounds also give a page some weight and make it more attractive. You can do all of these things without sacrificing clarity.
8. Motion Lines Are a Cheat
This one really just applies to me, I think; E.C Segar would look bloody awful without his blur of motion lines. But I like to think that, if I've got the pose right, if I've created credible body language, a thrown punch (for example) shouldn't need motion lines to sell it – the arc of the fist and its impact should be obvious from the characters' positions in the panel. I'm not above using motion lines to get a movement across, but I always feel like I've failed a little bit if I've found myself in a position where I can't sell that action some other way.
9. Don't Run Bleeds Into One Another on Facing Pages
This is really an offshoot of #1, clarity, I suppose. It's just something I try to avoid because it makes for a muddy, occasionally confusing reading experience – unless I'm drawing a genuine double-page spread, in which case it's essential, of course. But for the most part I try to make sure there's some white space between pages – so, for example, if I've got a bleed going off the bottom right of a left-hand page, I'll avoid having a bleed on the bottom left of the right-hand page (although bleeds at the top of that page would be perfectly fine). I just think it's clearer, and easier on the eyes, if one avoids having everything run into everything else.
10. End Each Page on a Gag or a Mini-Cliffhanger
I got this one from Carl Barks. I suppose it comes from the newspaper strip tradition, albeit in comics you can do it more subtly because you don't have to wait days or weeks between pages. It's a great way to make your pages hang together as a single unit; the comic reading experience has its own kind of rhythm, each page turn being a beat, and that rhythm contributing to your narrative momentum. Use that rhythm! Barks used to make it every half-page, because he worked in half-page units, but I think once a page is about right for a modern comic book (where, let's face it, there's about a quarter of the content of an eight-panel-per-page, 32-page Barks comic).
That's all I got! Not all of these will be right for everybody; it's a personal list, for sure, built up over the years to enable me to more effectively make my specific kinds of comics. But hopefully there's something in there you can use, or push back against, or possibly even tear down entirely and completely reinvent.
Anyway – hope it helps.
You can view Roger Langridge's work at: www.hotelfred.blogspot.com