In art, the human body is THE hardest thing to draw. And.. Um.. Horses.. :)

A bit of knowlege on the human body is helpful. In order to capture the human body in art, we must know what goes on underneath the skin. All artists throughout history have studied this. Learning to draw the skeleton is extremely beneficial in art.

There are books on the subject which can be useful aids for the novice artist.

More importantly, it's all about obsevation and practice too. Each person has distinct features. Try to exploit them while drawing. Do they have beautiful eyes, or a funny shaped nose/mouth?

Yeah, faces are always hard. (Well, next to hands, I've never gotten the hang of that.) Don't know where you're at in your own drawing, but I'll give you what I've picked up. (Five years of art school, and all I got was opinionated :)

Myself, my first big artistic leap was after I read How to Draw the Marvel Way. Don't use everthing from that now, but it really helped me out in the beginning, and I'd recommend it to anyone looking to go over the basics.

Also, anatomy REALLY helps with drawing people, and not just the head. Faces are easier if you know where all the muscles are and why they're there. You can pick up one of those anatomical models pretty cheap, and a good excercise is trying to draw various muscles and bones. (Plus makes drawing zombies and reavers alot easier.)

My other big advice: start rough. Do thumbnails to get the proportions right. You can do the detail-work later, and make the lines all pretty, but make sure the basic outline is proportioned right first. I'm from the school of making lots and lots of messy lines and tightening it up from there.

And don't forget: practice, practice, practice ;p There's been some conversation on this previously, but remember not to get discouraged. There's alot of really great artists on this site, but remember they all really sucked at one point :)