And also,

THIS.

More specifically, advice:

My advice is USE AS MANY CHEATS AS POSSIBLE.
No, serious.

For example, I no longer bother racking my brain to come up with poses for my characters of other people's characters, because I found three very useful resources: deviantArt , Stock photos, and Google Image search. If I'm really lazy, or can't get the pose right, I've even sometimes traced the photos on tracing paper used a lightbox to transfer it to manga paper.

And deviantArt also has a huge selection of well-done images of any character you could think of, if you'd like to draw one. EXAMPLE.*

My second piece of advice is SPEND LONG HOURS OF CLASS TIME DRAWING WHILE PRETENDING YOU'RE WORKING.
Okay, that's probably bad advice, but if you're good at double-tasking,what you do is use your sketchbook as your note book. So it is that on one page of one of my old sketchbooks, I have pictures of an OC and notes on the real probable causes of the Thirty Years' War. Finding the square root of fractions is mixed in with sketches of Hayner from KH II. The declensions of verbs are crammed at the bottom of a mini-comic about the lunchlady, who makes the worst pizza in the world-Wait-, I'm getting distracted.

Anyways, I have another tip:ALWAYS HAVE A SECOND COPY. That way, if you're working in something permanent like ink or marker like I do, when you screw up (like I do) you have another copy to do. And now that you know what not to do, thisone should come out better.
Uhh...that's basically it.

Oh! And don't forget to practice daily! It's taken me, what, four years just to get to this level, and I'm still not as proficient with markers as, oh let's say SOMEONE LIKE THIS. D;
or even THIS. D;