While forgetting about your legs and feet during the sprint is advisable. You can do certain drills that concentrate on them to increase the firing speed of your muscle fibers for a short period of time before your 40 yard time trail. Do about 2 or 3 sets of 10-15 yards where you take really short fast steps (it will kinda look like your doing half you normal height for high knees while really pumping fast) where you focus on hitting the ball of your foot directly underneath you and dragging your planted foot backwards over the ground with your hamstring deliberately pulling you forward.
Make sure to use your arms in a regular motion when you do this. He said it is to get the muscles used to firing right away when you hit the ground before your time trail or beginning of practice. You may get some weird looks doing this but I learned this from a professional track and speed camp coach. It subtracted .2 seconds off of my forty the first time I tried it.
Another tip to promote complete relaxation in your upper body is when you are practicing hold 2 saltines, one in each hand in between your thumb and your index and forefinger. You cannot break the saltines or else you are wasting energy gripping your wrists and fingers too tight. Remember complete relaxation of anything besides pumping your arms up and down parallel to each other and you feet.
Your fingers should look like two rings while the wrists stay completely relaxed. Make sure you arms are bent at 90 degrees and do not swing side to side at all since it will waste energy as well
Another great idea is prior to running to hyperventilate like a diver to super saturated you lungs and blood stream with oxygen. Breathe deeply in and out through pursed lips with you chest held up high and hands placed on your upper thighs. During this time, use your imagination and visualize the air saturating your lungs with oxygen.
Finally, when you take a video of yourself make sure you are planting your foot directly below you center of gravity when running. Too far ahead and your are essentially applying the brakes a little on every step and too far behind and you aren't getting full power out of your stroke
Edited: "It added .2 seconds onto my forty the first time I tried it." into what is says now. Brain fart