Be sure you know where along the JMT there are bear boxes. It will help greatly in planning how much food you have to get in your cannister. Obviously, the food you eat the 1st day out does not go in the cannister. If you have access to bear boxes for the next night or so, those days food do not go in the cannister. For a list of bear boxes,
http://www.climber.org/data/BearBoxes.html
For example, 3 of us hiked from Onion Valley, over Kersarge Pass and then on the JMT to LeConte Canyon and out over Bishop Pass - 6 days - with one cannister. We had bear boxes the 1st night at Kersarge Lakes and the 2nd night at Woods Creek Crossing. So we only needed the last 3 days of food for 3 people in the cannister. That is not difficult if you follow the packing regimen described by Tom Reynolds.
For tips on how to pack your canisiter, read The Care and Feeding of Your Bear Canister by Tom Reynolds. You can download that document from this pcta webpage:
http://www.pcta.org/planning/before_trip/health/bears.asp
There are great little tips the make a big difference: like not using ziplock bags, but instead using bags with twist ties and then putting the twist tie at the top of the bag so you can then squish the bag into any shape and fill little pockets around bulkier foods.
Turtle Walking