Last night I discovered this 34 Typographic Sins letterpress poster designed by Jim Godfrey and printed by Rowley Press. I read through all 34 and found it to be an excellent reminder! I know I've forgotten or gotten lazy about a few of these rules (NOT widows or orphans though! :). My fellow designer, Laura, and I are typographic-nazis, so this is right up our ally.
The best part? You can download the poster for FREE! It's a sort of "typographic mission work." Thank you Jim!
Download poster here