Here are some simple tips that I've picked up while drawing my own pictures, and noticing other pictures...
Always use pencil on your first sketch. It's easier to draw lightly with a pencil than a pen. Also, draw lightly so when you do make a mistake, you can erase it and it won't make a really bad mark on your paper.
Be flexible. Sometimes a picture won't end up like you want it to.
Have a solid idea in mind if you're trying to draw a specific thing. If you're just sketching random things, then let your mind flow freely.
Have a morgue. I know, that sounds weird. But if you have a collection of pictures that have different poses, hand gestures, body positions, then it is easier to draw those particular things because then you'll have a model.
Listen to music. I know that sounds cheesy, but I do my best work when I listen to music. And pick music that fits your picture. (For example, if you're going to draw a picture with fight scene, then listen to action music.)
If you have any good tips, then E-mail me and I'll add them to the list.
