My biggest problem with drawing comics is probably pretty common out there: it's how to not lose interest and give up just after the beginning. There are innumerable dead projects in my notebooks which fell apart after just a couple of pages. Pretty much the only certifiable successes I've had so far are my Improvisational Manga submissions -- where, of course, you're allowed to dispense with a lot of the elaborate preparations -- and No Headroom, which is actually about halfway finished and shows no signs of stopping (it may not be that good, but it's definitely gonna get done!)
So, here's the lessons I've learned about how to save time, maintain interest and get the damn things out the door. These may apply only to me, of course, but still. Okay, here we go:
1. Don't waste your time on setup.
The first and most important lesson. Drawing a manga page is an immense expenditure of psychic energy. Every page that you waste on boring setup brings the whole project one page closer to an irrevocable Apathetic Death Spiral. Ideally, the setup at the beginning of the comic is either so brief it fits on a few pages or (even better) in a few panels. You could also throw it out as a text-only prologue or even just flash back to it later. Either way, do the bare minimum of setup and get to the meat of the story as quickly as possible.
2. Don't lose sight of the good stuff.
Ultimately you're drawing this manga for you -- no, no, don't deny it... So don't forget to put in lots of stuff you like drawing. Whether that's fight scenes, intricate mecha, erudite dialogue, tense drama, attractive characters not wearing a lot of clothing, or (my personal favorite) mysterious, scary winged figures, bend the story so you can put it in as frequently as possible. Trust me on this one: it's vital for keeping you, the artist, interested in your creation.
3. Plan out the immediate future but let long range stuff take care of itself.
It's a mistake to draw a manga page without writing the script in advance and storyboarding it as well. But don't waste your time planning/writing scenes dozens of pages in the future. It just saps your energy from actually drawing the current pages, and by the time you get there your hard work will probably be obsolete anyway.
4. Keep it moving (or, Tell, Don't Show.)
For example, if your characters are spending six months in an arduous ocean crossing to get to the continent where the plot takes place, but nothing important actually takes place during the voyage, a great way of bringing this across to the reader is like this: a caption "Six months later..." and a panel of the ships arriving in the New World, with a character commenting that boy, that was a tough journey but here we are at last. Basically, if you're not interested in drawing some event, just have a character allude to it and keep going.
5. Murder your darlings.
This is a term from writers' workshops, referring to beautifully written scenes, characters, or turns of phrase that writers are loathe to cut even though they don't add to the story as such. Cutting these elements is even more important in comics than in prose. Sure, it may sound great now, but when it comes time to draw it, and draw it, and draw it... you'll be tearing your hair out halfway through from boredom. And if it bores you, think of what it'll do to your readers. Save these elements in notebooks and maybe you can make something else out of them later. In the meantime, cutting mercilessly lets you get to the "good stuff" earlier (see #2). As a corollary, if you find yourself writing basically the same dialogue twice -- for example, a situation that has to be explained a couple of times to different characters -- you're doing something wrong. Reshuffle things so everybody gets briefed at once, or maybe just drop it entirely and assume the explanations happened offstage.
6. Don't waste time explaining yourself.
It's hard to describe it well, but a certain sign of this problem is when you find yourself adding scenes to logically justify things that have already happened. If they are minor, unremarkable events that do make sense when explained briefly, then don't bother trying to justify them at all -- readers can and will do that for themselves. If, on the other hand, these are major events and your explanations require a lot of intricate logical tapdancing, it means something's seriously wrong with your plot. Take it back to the shop and fix it (which will probably involve #5, above.)
7. "Boy, these silhouettes sure save a mess o' drawin'..."
(As Walt Kelly said.) Take "labor-saving measures" whenever you think you can get away with it: dialogue-only panels or pages, splash pages, a few very big panels as opposed to many small ones, silhouettes, duplicated panels, Photoshop trickery. Just remember, if it looks good, it's not cheating.
There's surely a lot more to learn.
But that's all I know right now.