Cause of dew:
You have a local surface that is below the dewpoint. It has gotten enough colder than the air, that water condenses on it.
Dew is heavy on clear nights (more surface radiation cooling as your sleeping bag tries to warm up interstellar space) Smoke in the air, or overcast will reduce or eliminate the amount of dew.
Dew is heavier when the air is more humid. (Temperature doesn't have to drop as much for water to condense) However water vapour interferes with radiation.
Dew is almost non-existent if there is a breeze. The air next to a surface can't get that bit colder to allow water to condense.
Move away from water. Running creeks, lakes, swamps will provide a source of water vapour. Large bodies will be warmer than the land at night and will evaporate strongly, putting more water into the air.
Move away from the bottom of the valley. Cold air settles. I've found that being even 15 to 20 feet vertically (5-6 meters) can make a 10 F (5C) difference in temperature.
Put something between you and the sky. This blocks radiation from the surface of your bag, so the surface doesn't cool as much, reducing or eliminating dew. This can be a tarp, a tent, or the canopy of a tree.
Reduce the amount of visible sky This is a variation of the previous point. Radiation is going to be proportional to the amount of sky that your sleeping bag can see. Very little dew at the bottom of a cleft. A ravine can reduce sky by 50%
Dealing with Dew.
If you use a tarp or tent, pack the wet layer in a plastic bag so that it doesn't get your other gear wet, or roll it up in your foam pad to keep it out of your pack entirely.
Site your camp to catch the sun at sunrise. Or as soon as you get up move the wet item to a spot that is sunny. If I've tarped it, I will raise the west corners so that the entire tarp is exposed to the sun.
Get a lighter coloured sleeping bag/tarp/tent. You actually want a fabric that is very reflective in infrared. This radiates poorly. As far as I know no one makes such a fabric for backcountry gear.