Well, the typical art instruction always applies when it comes to humans:
Step 1. Find paper and pencil.
Step 2. Place pencil, pointy end down, on to the paper and move it around.
Step 3. Repeat steps 1 and 2 for 20 years.
As for more specific stuff, be sure you're not just drawing people and hoping they turn out right. You've got to draw from life, or reference, and you've got to study how the human body is composed and works; if you don't, your figures will always be stiff and unnatural. Yes, it's boring work, but it's what's necessary.
Some references I've found to be useful are:http://www.posemaniacs.com/blog/http://cedarseed.deviantart.com/art/Big ... y-15014442http://cedarseed.deviantart.com/art/Gui ... 1-32046017http://cedarseed.deviantart.com/art/Emo ... n-47118559http://snigom.deviantart.com/art/Male-A ... s-44663581http://snigom.deviantart.com/art/Female ... s-43675502http://imagec0rrupt3d.deviantart.com/ar ... y-69280033http://snigom.deviantart.com/art/Arms-a ... l-44397669http://coelasquid.deviantart.com/art/Ho ... -104342407http://timflanagan.deviantart.com/art/B ... al-4751669http://snigom.deviantart.com/art/Face-P ... l-44290362
[Warning for minor nudities]
Also, if you've got some money to spare, I would highly recommend:http://www.amazon.com/Dynamic-Anatomy-E ... 327&sr=1-1http://www.amazon.com/Best-Bridgman-Box ... 381&sr=8-1http://the-structure-of-man.blogspot.com/
And, most importantly, practice practice practice practice, do some practice, then practice, add a little practice, sprinkle some practice on the side, and don't forget to practice. Oh, and don't get frustrated; humans are infinitely complex and are extremely difficult to learn; just remember that each drawing you do brings your figures one step closer to perfection.