I just wanted to share, after several attempts at cleaning, what I found to be the absolute best technique for cleaning G-Shock crystals, which are often recessed. This may seem obvious, but I thought I'd share anyway in case it helps someone.
Take a bottle of lens cleaner (window cleaner will work if you don't have camera lens or eyeglass cleaner). Spray it on one end of a Q-tip. Rub this wet end all around the crystal, being sure to get in all the corners around the bezel.
Then turn the Q-tip over, and rub it with the dry end, going over it until all the moisture is completely gone, turning the Q-tip as you go so as to wick up all the moisture.
Crystal should look brand new, even in the corners.
Captain Obvious, I know...