Some important principles:
--Eliminate all bulky packaging--replace those bulky Mountain House packages with freezer bags
--Flexible items--no vacuum sealing which creates a hard brick.
--Squash everything--take a hammer to bulky foods like noodles (doesn't alter flavor), or ditch the noodles and substitute couscous (a compact form of pasta). Rice and couscous take far less room!
--Concentrate on high fat foods, like olive oil, peanut butter. For example, add 2-3 Tb olive oil to your dinners when reconstituting for a calorie and flavor boost.

The first day's meals do not need to be in the canister, since they will be consumed before the first night. This helps a bit.

A few more ideas here:

How to pack a bear canister

"gottawalk's" version of packing a bear canister

BPL forum--page down to the post by Paul Wagner (our balzaccom)