Draw other people's drawings.
Seriously. Freehand, of course, no tracing. Draw cartoon characters, draw people in figure drawing instruction books, draw famous artists' sketches...try to reproduce, line by line, just by looking back and forth, another person's drawing. At first you'll just be mimicking the lines, but eventually some of their technique starts to sink in, almost by osmosis. You develop a "feel" for the lines, a physical hand-memory like you learn writing cursive or playing piano.
It's much easier to copy drawings than photos or real life; reality isn't in outline, so you have to do some pretty high-level visual abstraction to make a solid object just "lines on paper." Copying what others have done before teaches you in time to see the way good artists see, and the rest is just developing your hand-eye coordination.
Copying the paintings of famous artists was a classic method to teach people art technique, in past years. There's a lost DaVinci painting known primarily from other artists' drawings of it, a big active thing with men on horses killing each other. You can see two of the copies in a book which is in the list I'm about to type....
Shiny's Favorite Art Books
(This isn't a comprehensive list by any means; there are a lot of recent books on drawing superheros and manga and such that I'm not familiar with and which might be quite good...I haven't had a lot of money for luxuries like art books in a few years...and when I did, it wasn't all THAT much money, so I was fairly cautious about art books; I looked for the most comprehensive and broadly applicable subjects so that I got "more bang for the buck" than from the specialized titles. )
Figure Drawing for All it's Worth by Andrew Loomis (THE classic, written in the 1940's, but still in print in a cheesy trade paperback edition, I believe... ) This one was actually my dad's before me. I learned to draw as a kid copying these drawings (Loomis had some other art books, which are not in print any more...if you see Andrew Loomis stuff in a used bookstore, snap it up, it's worth it).
Dynamic Figure Drawing, Dynamic Anatomy, Drawing Dynamic Hands, Dynamic Light & Shadow[/b](on shading), Dynamic Wrinkles & Drapery (on clothing) and the one that breaks the title pattern, Drawing the Human Head. Also probably anything else that may be by Burne Hogarth. He did the Sunday papers "Tarzan" strip for years, and I found him VERY useful - Dynamic Wrinkles & Drapery is the ONLY book on drawing clothing I had ever seen, which is why I bought it and discovered how good Hogarth's books are.
Drawing on the Right Side of the Brain by Betty Edwards - the modern classic that taught thousands of people who "couldn't draw" how to draw.
The Artist's Complete Guide to Facial Expression by Gary Faigin (this is where the drawn copies of the lost DaVinci can be seen, pgs 172-173, in fact). This is the last one I bought, it being the only subject I had nothing on at the time, and it's a great treatment. Drawing expressions with Leela's one eye doesn't seem so hard after reading this.
Hope all this helps some. Good luck!