It depends on what pest you're dealing with. Just make sure you don't apply Neem in flower, it will ruin the taste of the buds. Getting a good IPM program together is a very important part of growing. I use a three in one spray in veg that has Spinosad in it and I'm really good about only going in the grow area in clean clothes without shoes on to prevent tracking in bugs from working outside. Having a loupe or microscope will let you see tiny eggs, spider mites and stuff like root aphids that are too small to spot with the naked eye. Cleaning the grow between cycles with bleach or another disinfectant is always a good idea as well.