If you are like me (and I know I am) then you like to use one pack for everything (day hike, week long hike, hauling groceries from store, hauling gym gear back and forth, etc.)
Small amounts (say 5 lbs) carried in a 3000 cubic inch pack, however, will often jiggle and jostle around. I've experimented with various ways to temporarily reduce the volume of a large pack so small loads will ride securely (e.g. straps, drawstrings, bunji cords, etc.) Here's my latest effort and, so far, my favorite.
The back bag of this pack is sewn like a 2 foot wide by 3 foot tall envelope. A 2' X 3' piece of fairly stiff 3/8" blue foam is inside the pack. When the pack is empty the foam keeps the envelope fairly flat and reduces volume to near zero. As I fill the pack the foam bends and increases volume as required. The six photo's shown here show 2 views each of empty pack, pack with 5 lbs of stuff and fully filled pack.