Hills build mental and physical strength and are necessary to incorporate in your training especially when your goal race is hilly. But even if you are targeting a flat race, hills are still extremely effective for form correction.
1. Drive straight up and down. As you climb a hill, notice how your arms have to pump straight up and down and your knees lift a little higher. Your elbows are tucked in and driving straight back. Any side to side motion wastes energy and becomes magnified running uphill.
2. Lift those heels up. You are toe-ing off the ground to run up, landing more on your toes or the balls of your feet. Your heels may never even touch the ground when climbing hills as that slows you down. Hills are a great way to practice a forefoot strike and getting those heels off the ground. I should be able to high-five your heels if I'm behind you.
3. Faster stride rate. Hills may demand a shorter stride length but they will certainly make you increase your stride rate or cadence. Increasing our cadence makes us a more efficient runner so we can become faster over a longer distance on flat ground.
The next time you run hills, remember these 3 things. Pump those arms and knees straight up and down. Lift those heels up. Quicken that feet turnover.