I've been working my way through "Drawing on the Right Side of the Brain" by Betty Edwards. It's a classic drawing text that I read as a teenager, but I must have skipped a bunch of stuff.
I certainly don't remember ever walking around with a picture plane.
Have to say, if I can ever get used to the squinting it'd be a real help on larger proportions where I typically have trouble. For instance, I always underestimate how long a persons legs are (or I badly OVER estimate, either way for alien-looking results).
Above is a snapshot of all the random drawings. I just used printer paper and a set of pencils in varying hardness.
The book is aimed at people who haven't drawn since childhood, and especially those who are convinced they CAN'T draw. I like drawing and do lots of it, so I wasn't expecting huge breakthroughs. But still I was pretty pleased with my profile portrait: