Always carry one or two Q-Tips (cotton swabs) in your camera bag with any other cleaning supplies. They're great for cleaning the eyepiece of your viewfinder! Touch one end to your tongue and give the eyepiece a good going over. Flip the Q-Tip around and use the dry end to polish up. Much like with diving masks, saliva helps prevent fogging for those colder/wetter months. Lens cleaning solution works also if you have that along.