When collecting plants for either medicinal or culinary use there are a few basic rules to abide by:
Make absolutely sure that you have identified the correct plant! If unsure consult a plant finder book
Do not collect plants from areas which have been (or are likely to have been) sprayed with pesticides or insecticides, or areas which have been freshly manured.
Don't assume that because one part of the plant is medicinal/edible that all of the plant is useable. For example, collect elderberries and elderflowers, but not the leaves or bark.
Avoid collecting plants from areas where dogs have easy access to (near public footpaths for example).
Respect the plant and only collect what you need. When collecting berries do not strip the plant completely; always leave at least half for the local wildlife.