First, make sure that you understand how to start the 40 yard dash properly. With proper hand and foot position you will be able to decrease your time easily. Improper form will lead to slower starts.
Next, analyze the way the athlete starts within the first 10 feet. Proper arm action and knee drive is required out of the dig phase to properly accelerate your body forward.
Last, sprint all the way through the line and drive your knees through the resistance. Each resisted set will help build strength throughout your hip flexors. Hip flexors are utilized to increase stride length. After some resisted sets, move on to the unresisted segments.