Along with written tests, the physical fitness component is one of the most important aspects of your application. Some branches place more emphasis on it than others, and all have different requirements, but it's important that during the time after being accepted and attending OCS that this be your number one priority.
It should also be noted that the training plans aren't exclusive to one branch or the other. All but the Marine Corps test for the same 3 events, so it's easy to find a plan to utilize training for one branch on another branch's site.
Army
Army PRT - the ultimate PRT information source, from background to technique to training methods and workout plans, as well as scoring systems - http://www.armyprt.com/
Navy
PRT Standards - contains tables breaking down scoring by gender and age, as well as minimum scores. Also contains links to several other Navy and fitness related websites and training plans - http://www.navy-prt.com/
Overview of Navy PRT - Explanation of PRT, scoring system, and several links to training programs, score tables, and other fitness sites - http://www.military.com/military-fitness/navy-fitness-requirements/navy-basic-training-pft
Air Force
Overview of Air Force PFT - Explanation of PFT, scoring system, and several links to training programs, score tables, and other fitness sites - http://www.military.com/military-fitness/air-force-fitness-requirements/air-force-basic-military-training-fitness-test
Air Force Personnel Center - another brief overview of PFT - http://www.afpc.af.mil/affitnessprogram/
Marine Corps
Overview of Marine PFT - Explanation of PFT, scoring system, and several links to training programs, score tables, and other fitness sites - http://www.military.com/military-fitness/marine-corps-fitness-requirements/usmc-physical-fitness-test
Ultimate OCS Prep Workout - workout plan put together by a former OCS attendee with daily workouts - http://officercandidatesschool.com/blog/2013/03/07/the-ocs-preparation-workout/
Marine Recruiting Fitness Recommendations - brief overview of some ways to improve the physical component of your application - http://www.marines.com/becoming-a-marine/ocs-physical-prep