The more humid the air, the higher the dew point and the less likely that frost will form on those plants. Heavily mulched plants are more likely to be frosted, since mulch prevents moisture and heat in the soil from escaping and warming the surrounding air. Know your plants. The plant itself determines the likelihood of frost damage. Frost tolerance tends to be higher in plants with maroon or bronze leaves, because such leaves absorb and retain heat.
Compact plants expose a smaller proportion of their leaves to cold and drying winds.
By the same token, closely spaced plants protect each other. So you've checked the weather conditions and decide that, yes, Jack Frost is coming and protecting your plants is worthwhile. Spun-bonded fabric row covers will protect plants down to 30F, polyethylene row covers to 28F. Cover the whole plant before sunset to trap any remaining heat. Lightweight coverings such as row covers and newspaper should be anchored to prevent them from blowing away. Second, keep the soil moist by watering your plants the day the frost is predicted.