There are many different ways to push for greater rights and access for off-road cyclists.
One of the simplest is to make sure that the trails you ride are kept clear of obstructions. While bigger obstructions, such as fallen trees or locked gates, should be dealt with by contacting your local authority, small branches and other obstacles can be easily dealt with at the time you encounter them.
Another simple, often an overlooked way to support mountain biking is to encourage your friends and family to get out on their bikes. Getting more people cycling makes local authorities realise the importance and popularity of off-road cycling, which encourages them to spend more money on facilities and trails.