Lenses really need to be kept clean and free from contaminants to remain as water repellent as they can be... wiping with fingers is about the worst thing you can do because the grease on your fingers is hydroscopic, which means it attracts water even more. I use a blower in the field, and save the cleaning till i get home, using cleaner designed for optics. The lens wipes you can get in little packets from tescos etc seem to do a good job if you've got an in field emergency, but all the same, blowing and cleaning is something for the clean dry and relatively dust free environment of the home for me. I find that way i've little problem with fogging at all.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep