Most people who visit this forum are familiar with Bridgeman, Loomis, Michael Hampton, Burne Hogarth etc. These are the great, well known methods, but I thought I'd share some drawing techniques I've come across that aren't easily found anywhere else.
1. Drawing without inner lines. This is a great exercise for learning to draw in three dimensions, it's a challenge feeling the forms and it will help develop a sense of depth and solidity.
2. Drawing cubes without inner lines, and moving them through space. Similar to the first exercise but also helps to develop a sense of perspective.
3. Wireframe. Another method of drawing form, is to draw cross sections of the form and then connect certain points of the cross sections with lines. It's fairly easy this way to draw forms that are really complex.
I learned these techniques from Mark at le-mec.deviantart, or moatdd on twitch.