Help your local mountain bike club maintain trails. If there isn't a volunteer effort already in progress in your region, start one. Focus your trail work on problem areas, such as steep switchbacks or low-lying areas that have poor drainage. Invite the local newspaper to write about and photograph your efforts. Quantify your volunteer hours and always make sure your work is approved by the appropriate land management agency and coordinated with other trail user groups.
Many state racing series offer points for trailwork.
Share the joy. Introduce someone to the wonders of mountain biking. Teach the sport to a friend or even a local land manager. A land manager who rides and appreciates mountain biking is an invaluable ally. Bike shops are often willing to donate a bike to a land manager, especially when the recognize that a positive-minded government official usually means positive mountain biking policies.