There are a lot of how-to books for drawing and even making comics. I've read a few and most seem to be lacking in practical advice. Things like,
Often, the oldest advice is the best.
If a book has been in print for 20, 50, 80 years, good bet it's what the newer books are based on. Norling's Perspective Made Easy, is a good example. I've flipped through a few books on perspective, and that one is best, in terms of comprehensive and easy to follow. I think some of these writers mistook themselves for Pythagoras.
When you're starting off, work cheap.
Those fancy comic boards with the measurements marked off and printed are nice, but they end up about a dollar a board. When you're starting off and just doing practice stuff, it's better to just go get a ream of 11x17 paper from the copy shop. The measurements on those pre-printed boards help a lot, so maybe get one pack and use them as guides, using masking tape to tape down a piece of the regular stuff. Or you can find some templates, online, to print out (like these http://elfkin.deviantart.com/art/Comic-template-kit-1-215257523 ). If you're planning to paint on it, then you'll want a book of watercolor paper (or something similar) and to use a lightbox. Which brings me to the next thing...
Learn how to build your own lightbox.
A good lightbox costs a small fortune. But building one is not big deal, because it's really just a wooden box with a Plexiglas top and a light inside. If you're not secure in your construction skills, you can jerry rig one. A couple ways to do that: turn an empty fish tank on its side and stick a table lamp inside, or get a glass top drafting table and get a flat light to put under it (like the kind they put in fish tanks).
Kill your ego (and don't take it personally).
When someone offers you a constructive criticism, they aren't trying to make you feel bad, they saw some sort of potential there, and want to help you get to the next level. If your stuff is just that bad, they won't bother criticizing. Sometimes, the only way to learn, is to fail. So if you failed, get over it, and don't do it again - learn from your mistakes. But when your failure isn't obvious, it takes someone with experience to see it. One of the biggest favors you can do yourself, is to kill your ego and listen to advice from someone who's been there, done that.
Don't be afraid of the pros.
They had to start somewhere, and if they see a talent, they may just latch onto it and try to guide it to the next level. That's what happened with Neal Adams and Frank Miller - Neal recognized the potential that Frank had, and tried to guide him, and eventually helped Frank get into the industry. And don't get discouraged because your favorite artist didn't respond. Often your favorite artist is a lot of people's favorite, and just doesn't have the time to take you under their wing. Most comics artists are really nice people and they'll help you, if they can, but they just don't have the time to go over everyone's portfolio and give them critiques.
Special thanks to Eric.