John Brandt’s Special Character Shortcuts (free) is a comprehensive PDF that shows how to access almost any special character in any font. The 16-page PDF includes cheatsheets for accented characters, ligatures, punctuation, currency, legal symbols, math, and greek symbols. Print out the pages for the characters you forget most often, and post them near your computer.
Link: Special Character Shortcuts PDF
Keystrokes are listed for Mac and Windows, as well as HTML codes for web pages. There is also a special section on special nonprinting characters in QuarkXPress 7 and 8, and a handy typographer’s glossary.