For prevetion I like to use lots of sticky cards, I put them everywhere these are especially good in my indoor grow's they are realy effective in dealing with Fungus Gnat & Thrip. My outdoor grows often get invaded by Aphid, Spider Mite, Scale, Mealybug, Whitefly, Caterpillars & Grasshoppers. For Aphid & Spider Mite I first try to use the garden hose and spray them off with water this sometimes is sufficient enough alone and a big thing to look out for is Ants they are trouble they will bring the Aphids & Scale on to your plants they work in a symbiotic relationship with them the Ants offer protection while in return the Aphids & Scale secrete a sweet honeydew like substance that the Ants eat so get rid of the Ants they will keep bringing them to your plants or move your plants to an erea free from Ants. If the infestation is to great then I like to use an IPM Spray, the one I use is from Organic Gardening Solutions (OGS IPM SPRAY) it contains Emulsified Neem Oil, Silica, Saponins, Fulvic Acid, EM1, Aloe & Kelp Ferment, Essential Oils & Aloe. This type of IPM Spray is very effective against all that is mentioned above but the best prevention of all is to inspect your plants everyday so you can act promptly before an infestation can occur.