1. If there are several entities in a busy part of the drawing, eg a square and a circle and a stack of planes and other lines;   and you want to select say the circle,  then you can click near them all, and cycle through them using the Enter key.

2. If you want to change the seniority of a line, then you can delete it, and then press Ctrl+Z to resurrect it (but now more modern).

3. One quick way to draw a line normal to another in 2D (or in 3D) is to use parallel command (=) to draw a second line, and join the ends of the lines.

4. When drawing a vector, or a balloon or a similar cranked dimension, say a diameter, press Shift as you set the final point, which will reliably give you a horizontal line (whereas Ctrl won't reliably give you a vertical one for instance - due to "mouse wander").   

5. PS Likewise when sketching a sphere in 3D, press Shift as you set the second point to get an orthogonal sphere.
etc etc .