First, keep at it. I've been doing GTD for several years, and it just takes time to "get it".
Originally Posted by propynyl
If you really want to have a next action after a shopping trip, maybe it's "brainstorm stuff to buy at the store".
But Katherine's advice is good: maybe you need a collection bucket that is a had written list in the kitchen. Not everything needs to be in Omnifocus
I use a combination of the handwritten list, context specific purchases in lifebalance, and splashshopper on my Treo.
Omnifocus - iPhone - iPad - BusyCal - Spacepen - Field Notes notebook