As they begin to flower, reduce watering. Increased trichome development has been associated with drought conditions. Don't stop watering, but reduce it. If you've been watering every day, try going to every other day. If you've been watering weekly, just reduce the amount. If the weather is substantially cooler in September and October, I'll sometimes stop watering, for weeks before harvest. This assumes you check your plants, daily. I don't know your situation.