One of the nice things about a Newtonian Telescope is that the whole body of the scope acts as a dew shield. Another place that dew seems to form is the eyepiece. Many newer eyepieces come with a raised rubber piece surrounding the lens. This filters out extraneous light, but also acts as a mini dew shield. Sometimes none of the above will be available, the only thing left to do it heat your optics.
Excess moisture can eat away at the coatings on the optics and rust the interior of the telescope.