1. Respect the law. You must have permission from the property owner to collect plant matter. To forage without permission is considered stealing and you can be arrested. Most state and federal land prohibit gathering plants except in survival situations.
2. Respect the land. Leave no trace. Fill your holes, pack out your garbage (and garbage left by others), don't hack/slash/smash/burn your way through nature. Don't harvest a plant if there are just a few around.
3. Respect the plant. Please harvest sustainably so that there will be plenty of plants year after year. I've coded each plant with an Abundance tag which are defined as follows:
Plentiful - harvest up to 50% of this plant from the location
Common - harvest up to 30% of this plant from the location
Uncommon - harvest up to 10% of this plant from the location
Rare - harvest only in an emergency or one small bit to taste
Very Rare - harvest only in a dire emergency
Also, don't strip all the leaves from one plant, just take 1 shoot or 2-3 leaves from many plants. Minimize damage to the plants by cutting leaves off the plant with a sharp knife or shears rather than tearing them off. Harvest inner bark using long, thin vertical strips on one side of the tree, do not cut a ring all around the tree which will kill it. Sterilize your cutting tools with alcohol or bleach to prevent transfer of diseases.
4. Respect yourself. Please positively identify any plant before eating it. Eating the wrong plant can lead to illness or in rare circumstances, even death. Also be aware of any environmental hazards in your foraging location such as snakes, bears, or chemical hazards from old oil fields, roadways, lead paint around old buildings or areas subject to flooding from sewers.