I create a playlist when I start developing the idea, that basically creates the identity of the film in my head. When I hear those songs, it puts me in that frame of mind. I write to that music, too, so it all just gels.
Project 1 (an indie road-trip comedy): lots of indie guitar/folk music; some indie movie scores (Little Miss Sunshine, etc.)
Project 2 (a comedy of manners): mostly classical scores, but with some hint of lightness (The Shape of Water is a good example)
Project 3 (a political thriller): Trent Reznor's music, mostly. Dark, grimy and gritty songs.