You should research for yourself the specific deadlines for each program you are applying to (and remember to figure in time to register for tests and have scores submitted, for recommenders to think about and write letters, and so forth), since they can vary widely. But this checklist (which assumes an application for the fall after your senior year) should serve as a rough guide as you construct your own personal timeline. If you feel you are already behind, there's no need to panic - just get organized.

Junior Year and summer (or 15 to 18+ months before you want to begin school)
   » Start researching programs you might be interested in. Your search should be broad to begin with.
   » Begin to explore possible sources of financial aid.
   » Register for any standardized tests you will need to take to be admitted.
   » Take a practice test (or preferably several practice tests, and use a study guide).
   » If you are ready, take the test.
   » Decide whom to ask for letters of recommendation.
   » Check your unofficial transcript to make sure everything is in order.
   » Write a rough draft (or drafts) of your statement of purpose.

Fall of senior year (or 9 to 12 months before you want to begin school)
   » Meet with faculty to request letters of recommendation.
   » Have various people read your statement of purpose, and write a final draft.
   » Complete your applications and (if paper) mail them. Allow plenty of time.
   » Ask the registrar's office to forward your transcripts.
   » If necessary, apply to programs for fellowships or assistantships.
   » Check with programs to make sure your applications are complete.

Spring of senior year (or 6 to 9 months before you want to begin school)
   » Complete and mail FAFSA and any other required financial aid documents.
   » Make sure your schools have a number where they can reach you with their decision, so that you don't miss out on an opportunity.
   » Visit campuses, if feasible. Talk to faculty and students in order to help make your final decision.
   » Once formally accepted by the program of your choice, contact other schools to decline acceptances. (After April 15, you will need a written release from a school you've accepted if you change your mind.) Celebrate! (If you should not be accepted, decide if you can - and want to - improve your application for the following year, and consider other possibly less competitive schools, providing they are still satisfactory.)