Most of the time all it takes to get them off is enough water, applied long enough to soften them up. I usually get them off wet, with a soft carwash brush.
You can do the job faster, a bit more work, with soft abrasive materials like dryer sheets, magic eraser products, etc. Even the softest abrasives, however, will likely scratch waxes and other protective coatings, or plastics and paints.
Some bugs, love bugs particularly, can etch into coatings as they bake, and once they do that, they are harder to remove.