Generally washing herbs is just what it sounds like, run it under the tap and pat dry, but water changes with every new experience, there are a couple of documentaries that show that the molecular structure of water changes depending on what happens to it, tap water looks banged-up and ugly, and water that has been thanked or blessed takes on a much more snowflake-like structure. So if you wanted, for extra power and niceness to your potions, you could wash your herbs in a bowl of water that you have infused with your intent, either through chanting, meditating or even simply speaking to it :) Would recommend a bowl made out of natural materials as well, or one you've made yourself :)
I would add to make sure your herbs are completely dry before making potions and tinctures, especially if you plan to store them for a time. I tie small bundles and hang upside down, or place on paper towel in a basket to thoroughly dry them.
One thing I highly recommend: wash all herbs in filtered, spring or distilled water. Do not use straight tap. Plants still absorb a while after being picked and tap water is loaded with chemicals. Herb preparation is different for each herb. Cleaning however just takes a good rinsing. Pay special attention to where the plant has any grooves, and the underside of leaves as little bugs and larva attach there most