Club T-shirts are great if they are clean and the message aids your argument, but watch those gnarly graphics. Find common ground. Look for an issue you can both agree on and work from there. Perhaps key concerns are trash dumping, park budget cuts, or the threat of development adjacent to the park. Ask how you and your group can help the park fight these threats.
Nothing gets attention better than up front honesty and the ability to move past disagreements.
Don't get stuck on obstacles. Instead, make a note and come back to the issue later if you think it is worth your time. Understand before you seek to be understood. This is an old proverb, but it really helps if you understand the system, priorities, and constraints the land managers must deal with. Follow Up 9.