+1 And quite often the washer'll be stuck to the o-ring....a thin pick tool with a little hook bend on the end. Keep trying till you get it....it'll be in there, even if it looks like it isn't!
I usually get them out by wrapping a little masking tap around the blade of the screwdriver. As was already mentioned, be very careful as to not lose the tiny washers and o-rings. If you go the compressed air route, be sure to remove them first.