Commercial shower cleaning treatments such as Shower Clean, Shower Shine and Mist Away prevent soap scum buildup by creating a film over the shower surfaces that water drops won't stick to. The "sheeting" created when water hits it will also keep ceilings relatively free of water droplets. Clean the ceiling thoroughly before applying the shower treatment. Wipe with a soft cloth, and allow it to dry. For best results, apply the cleaner to the ceiling after every shower.
A textured ceiling won't repel water, but it will make it difficult for water droplets to form. Texturing a flat shower ceiling is the same as texturing any ceiling with joint compound. The best textures for preventing water droplets are rough or grooved, which you can create simply with a long-nap roller over wet compound. You will have to wait a couple of days to use the shower after texturing the ceiling, but the results will require little maintenance.
If you're doing a full renovation of a bathroom and are looking for a preventative for water droplets up above, design the bathroom with a tilted ceiling, at least over the shower area. When the water drops hit the ceiling, they'll slide down the ceiling to the wall, and finally into the basin and down the drain. This works best when you treat the ceiling and walls with shower cleaner.
The less steam that rises out of the shower, the fewer water droplets build up. While cold showers are probably not worth the payoff of a dropless ceiling, keeping the temperature at a comfortable setting and avoiding running the hot water running too long so the bathroom doesn't become thick with steam can reduce water droplets. If you're unwilling to turn down the heat, keep a long-handled squeegee on hand in the bathroom to wipe away the droplets as soon as you're done showering.