Arguably the most important act is help insure safe tap water for all. Several nonprofit organizations are working to give the Clean Water Act teeth. The two most readily Googleable are Clean Water Action and Clean Water Network. Contact them and inquire about volunteering, donating, or supporting their campaigns in other ways.
Second, research your local tap water. The New York Times has compiled several resources on its How Safe Is Your Water? page, including a database to help people identify water polluters in their area. Consumer Reports, our beloved sister-site, also has some handy info for analyzing your water. You can use this information to urge elected officials in your area to hold polluters accountable.
In the mean time, use water filters. A few online guides can help you choose one. The one at Water Filter Comparisons is based on performance claims provided by the manufacturers that have been certified by the NSF, UL or the California Department Of Health. Consumer Reports analyzed filters in 2007, which subscribers can find online here.