Dr. Boyce has a great website with advice for students considering graduate school.
Check out the application requirements of the schools you might be interested in well ahead of time (i.e. late in your junior year). This gives you plenty of time to get your application packet together. Remember, taking the GRE and getting your scores back may take longer than you'd think.
Consider applying for an internship before you go to graduate school. It's a great way to gain experience, meet new people, and go new places. It will also look good on a graduate school application. There are lots of opportunities available, including the following...
Student Conservation Association
American Public Gardens Association
The Smithsonian Institution