Stick figures are good to start with -- figure out where the head & spine are and where the joints of the arms/legs are before fleshing it out. If I'm drawing something especially difficult I'll turn the stick figure into a rough skeleton and add muscles, etc.
Get a good anatomy reference book (mine is Peck's Atlas of Human Anatomy but there are lots of others). There are mathematical formulas you can use to rough out the figure - i.e. a normal adult body is about 7 heads high, etc.
I can recommend a book called The Artists Guide to Facial Expression (Gary Faggin) -- it has a great tutorial on constructing a realistic head from geometric shapes. This was the one I used to create believable faces from scratch, in the correct proportions.
My favorite technique in spotting mistakes when you don't have a reference image to look at your work through a mirror. Other things you can try are turning it upside down (as mentioned above) or 90 degrees, with feet at bottom and head at top (especially good for reclining poses). Also, move as far away from the image as you can and squint to see lights/darks.
Drawing from life is in my opinion the best way to create better imaginary figures, because you'll get a better sense of innate proportion than copying a compressed 2-dimensional image. One of the most useful life drawing books I've used is Kimon Niccolaides' The Natural Way to Draw. He recommends this daily exercise: Draw a 5 X 7 inch image, with a pen or pencil (no color/shading) of something you remember seeing in the last 24 hours. Draw for 15 minutes and try to reproduce every detail you can remember.
Again, the mirror is a really useful tool -- if you want to figure out a hand gesture or facial expression you can use yourself as the model -- not to copy the image but to see what an frown does to the cheeks and eyes, or how a fist looks from an angle.
Here's a link to a tutorial in gesture drawing -- AKA making a very quick sketch to capture the essence of what the figure is DOING, rather than worrying about finished appearance: http://www.slideshare.net/honoria/gesture-drawing-introduction
Gesture drawing is easy to practice in public places or while you're waiting, and often gives you poses you wouldn't have thought up yourself.
It's as much about training your eye to really observe how muscles and bones work together, how gravity works, how emotion affects a movement, etc. The reason some of these famous pictures work despite them being a little out-of-proportion is that the artist usually conveys the intent or the movement of the figure. Look at Rembrandt's sketches of babies for some beautifully observed moments with a few lines of drawing.
while I was looking for the URL above I ran across a Pinterest Page with some good examples of dynamic figure sketching and reference guides for character design and fabric detail: https://www.pinterest.com/pin/461619030538728573/