I keep a cheat sheet of Alt codes by my desk at all times. The whole grid of 8 bit codes and selected four digit codes. The ones I use most often are
½ (Alt-171)
° (Alt-248)
² (Alt-253)
± (Alt-241)
÷ (Alt-246)
× (Alt-0215)
≈ (Alt-247)
There's a huge wealth of characters using the four digit codes, far too many to include on a simple cheat sheet. Notably lacking are 'subscript n' and 'subscript i'.
I really should acquire and learn paint.net. The quick sketches that some have seen me post here and there are usually done in MS Visio, which is darn useful, but damn expensive. (I have Visio 2003 bootleg at home, and up to date Visio at work that I didn't have to pay for either.)