What's the best way to paint an acoustical ceiling? That depends on whether it's ever been painted before. Without paint, it tends to be very water-soluble. This means that painting it with a roller can be very tricky because the acoustical material will absorb the water from the paint and start coming off on the roller. To test the water solubility, spray a small amount of water on an inconspicuous area. If the texture becomes soft, the best way to paint it is with a spray gun.
If that's the case with your acoustical ceiling, use a good, vibrator-type paint sprayer (available at home centers). Since you'll be working overhead, you'll need a suction set and portable paint reservoir for the sprayer instead of the usual paint jar. The one we used has a backpack type of setup. To ensure a uniform coat, it's wise to apply the paint in at least three directions.