When I forget a key signature I sketch the above circle of fifths:

1. Make a circle with the notes going clockwise in perfect fifths (7 half steps) starting with "C" top center - C,G,D,A,etc...)
   
   
2. Draw a circle around "C" for figuring out major key signatures and a circle around "A" for figuring out minor key signatures. Use the Sharps-right Flats-left rule for figuring out how many sharps or flats in a key signature.
   
   
3. Draw a line inbetween the "F" and The "B-Flat". The word Sharp has a "R" in it so you go right to figure out the actual Sharps used in a given key signature
   
   
4. Draw a line inbetween the "F-Sharp" and The "B". The word Flat has a "L" in it so you go Left to figure out the actual Flats used in a given key signature