Cover your finder while you're not using it. Chris Sarnecki puts a knit cap over his Telrad while not in use and it stays nice and dry. Also, be careful that your breath does not get near the eyepiece while you're observing. Things like SCT corrector plates, Newtonian secondary mirrors, and refractor objectives. But you can cover them sort of.
Dew shields are simply an extension of the telescope's tube beyond where it normally ends.
It reduces the degree to which the corrector or lens is exposed to the sky. A dew shield can be made of most anything - plastic, waterproofed cardboard or poster board, metal, or foam rubber. You could even tape on a paper bag in an emergency. The thing to remember is that the greater the insulation factor of the material the better. The drawback is that dew shields usually don't work forever - they simply put off the inevitable, especially under damp conditions which are frequent in Michigan.