Ways to Help
Take it with you. Don't ever leave trash behind on a trail. The term is "Carry in, carry out". And it's simple to interpret, if it comes in with you, it needs to leave with you.
Stay on course. Trails are set up for reasons. They aren't there just to guide you to a destination but to also protect the natural environment around you. Any time you go off-trail you risk disturbing natural habitats.
Cleanup after OTHERS. This one can be a drag. Let's face it, no one likes picking up others' trash but it's so easy to throw an extra trash bag in your pack and make sure that trails stay pristine.
Research your hikes. It's important to have an idea where facilities are and how far you need to make it between pitstops. A little reading will arm you with knowledge on where to dump trash. It's also always important to know which local wildlife you'll be sharing the space with and what plants to avoid contact with.
Make a donation. Parks, forests and trails need funding. Make a contribution to a national, regional or city park. This pays for cleaning and offers facilities to hikers and birdwatchers while keeping the natural space as undeveloped as possible.
Volunteer. That's right, get out there and get dirty for the cause. There are many ways to volunteer; you can do cleanups with parks departments (both local, state and federal), you can provide education to children on the risks of forest fires, or you can offer yourself as labor to clear paths from trees and other objects that can obstruct trails.