Foy's Law of Necessity

This is how it goes:

While preparing for a shopping excursion or while packing a bag, make a list of everything you need.
Then, cut it in half. Cross off half the list.
Now, cut it in half again.

The Law:  You only really need 25% of what you think you need.

This works especially well when dealing with limited resources (e.g. budgets) and small bags. Good luck!