When harvesting herbs the most important thing is to ensure you have identified the right plant. A good field guide which I recommend is Wild Flowers of Britain and Ireland by Blaney, Fitter and Fitter.
Harvest only what you need, always leave some plant so it will grow back. When picking parts above ground, only remove the top half to two-thirds. Never harvest a plant if it is the only one in the area.
Use a carrier bag or basket, and bring gloves and a scissors. If you are harvesting for roots bring a shovel or digging fork.