As WashougalWonder noted, Castile soap works best. I have used this very successfully with no damage to the plants at all. Mix ~1-2 tsp of the castile soap in 1 qt water and pour into a clean spray bottle ( make sure it hasn't been used before to spray say windshield washer fluid or some other chemical )
Spray from underneath the plant on as fine a mist as you can get the thing to make ( Zep makes a really good quality spray bottle ideal for this purpose ) and coat the leaves. The castile soap works on soft bodied pests like mites and aphids by disrupting the cell membranes of the organism ( think the wicked witch of the West in the Wizard of Oz when she got dosed with water; it has just about the same effect) and stops them right in their tracks.Its non toxic to good bugs like lady beetles ( who devour mites like they are popcorn shrimp ) and doesn't produce resistant strains of mites the way commercial cholinesterase -inhibiting insecticides do. If the plant is flowering when you spray, just rinse the buds off with plain water after you harvest.