Ensure your plant and buds are healthy - To produce high THC weed you need to make sure your plants are healthy and happy. By avoiding the most common mistakes such as over and under watering, heat stress, irregular light cycles and nutrient issues you will massively improve potency, yield and flavour of your buds.
You should also try to replicate a plants natural environment as closely as possible; this means keeping tabs on the climate in your grow room which is easily done if you get yourself a good thermometer and humidity monitor.
Use Good quality grow lights - Although cannabis plants can grow in low light conditions both using a good quality high power grow light will not only help you to produce high THC weed but also massively increase yield.
High Power grow lights or simply growing outdoors in summer will help to increase THC along with the other cannabinoids found in marijuana.