I saw Dave Gray demonstrate his visual alphabet at SXSWi last year. If you haven't seen him demonstrate this, you might want to watch this video now. In the video he illustrates for us how sketching is just combining forms and lines, and all you really need to be able to come up with pictures is to see and use these basic shapes.
See also Dave's Basic Rules for Napkin Sketching after you've seen the above.
I remember drawing as a kid, feeling frustrated sometimes because I couldn't make things look the way they do in real life. I didn't really understand representational drawing until I started painting. In examining volumes, I was able to find a more basic understanding of the 2 dimensional shapes, and began drawing with ease. But it all came together when I took a Manga drawing class and started drawing characters that I really saw everything in this world of 2d and 3d shapes and polygons.
Drawing representationally shouldn't be the goal in sketching. It should be capturing an idea with rough, basic shapes. I think wanting to draw representational renderings is what held me back so much when I was younger, but embracing the basic shapes is what propels me now. After a while, you learn the alphabet and the pictures come easily if you continue to use the langauge.
I have a feeling these videos will come in handy very soon as tablets flood the market. Brushes and SketchBook Pro for the iPad will certainly provide another way for us to sketch.