Don't do it too often. Maybe every couple of weeks? If you see spider mites appear (they come along when plants are ailing) then you can alternate with a foliar spray. How big is your garden? Is a gallon of anything too much to mix? A gallon of water with a couple of ounces of compost tea (look it up), a couple of tablespoons (about 1 ounce) of orange oil (the strong stuff, that costs about $20 a pint). Put it in a backpack sprayer or a little of it in a spray bottle and spritz your tomatoes (and anything else that gets red mites) every three days.