America’s National Park Service is more than 100 years old. Yellowstone was the very first national park, recognized by Congress in 1872. But the agency didn't become official until President Woodrow Wilson signed the Organic Act of 1916, in the spirit of conservation and environmental awareness. The Park Service has since grown to include more than 400 national parks, monuments, battlefields, recreation areas, and more. Here are 19 tips for saving money on your next visit to a national park.