It's not something that you have to do every day...it's something that you compile over several years. Committment to a volunteering service that you spend 4 hrs/week with looks much better than cramming everything into a couple of months. As for shadowing, holiday breaks and summers are great opportunities to add to your hours. I spent fall break shadowing a general surgeon and will be spending spring break shadowing a trauma surgeon...each break adds about 30 hrs onto my total hours. Basically, don't wait until the last minute to get in your EC's.