A good start is to have a clean lens and not touch it with your fingers. Grease, dirt etc are all hydroscopic, they attract water. It's the same reason you see hand wipe marks on cold damp days on your windscreen.
If you use a good optical cleaning solution then you may not need any, although i'm saying this using a long eye relief scope. I've been using the optical wipes from opticians which come in the little tear open sachets. They work well in the field as you can keep them sealed and only use them when needed. The liquid evaporates so you don't need something else.
For rain I use a rocket blower, like vinny is suggesting, to blow the rain drops off. This gets down the scope enhancer and means i don't need to touch it in the field.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep