First, get rid of any sticker, shirt or hat that makes a joke out of the word "balls". "Got Balls?" isn't clever or even funny. Enough said.
Second, don't treat paintball like some sort of survivalist training program (for the woodsballers) or hip-hop concert with guns (for you speedballers).
Third, talk to people about the sport in a meaningful way. Many people that have no desire to ever play the sport are still curious about it. Be willing to share what it is and explain misconceptions of what it is not.
Fourth, recognize that paintball is never going to be a mainstream sport. Paintball simply isn't a spectator sport and it can't be played by any kid in the country. That means that there will never be fantasy paintball and that not everybody will be able to play it growing up. When you add in the cost of playing, even fewer people will ever truly embrace the sport. But remember, that's okay. It doesn't have to be covered in primetime to still have a devoted following.
Finally, use common sense. Unfortunately, many people that have a negative view of paintball and justify it thanks to all the vandalism that occurs using paintball guns. Keep your equipment out of the hands of irresponsible people and don't be part of the problem. Something that might seem like a good idea to do with a paintball gun that doesn't happen at a field probably isn't.
If paintballers want to be taken seriously, they need to earn it. Please help the effort.