If your using it on spider mites, add some pure soap flakes to the mix, this will remove the waxy coating they have on their bodies, and allow the alkaloids to penetrate and kill them. Sounds cruel, but they can destroy a garden. This can also be used on thrips as well.
Spider mites love dry conditions, so if they are in a pot plant, you can soak the pot plant in a bucket of water, for at least 6-7 hours, this will see them off. If they are in the garden, keep the soil moist, use a wetting agent if your soil repels water. The soapy water from dish washing, makes a good wetting agent.