Honestly, I think that the best advice, and probably the first one, is that you need three things:
1- practice
2- perseverence
3- practice
That means that you need twice as much practice than perseverence (shamelessly ripped off from "Chicken Run").
Then again, that's true in almost everything in life. Driving, school, animation, cooking, etc...
Otherwise, look at drawings, and try to see how they are made.
I'm not saying I'm a particularly good artist, but seriously, that's what I've always been told, and it's true... So, I guess that there's always this...