My Coach, Bob Sevene, has always been a big proponent of running form, and it has rubbed off on me. My form has improved over the years thanks to lots of drills and feedback from my Coach, but it is still something I like to focus on while running daily. Marathon training in particular makes you tired and can end up giving you the lazy marathoner shuffle if you are not careful. Have a friend video you if you are unsure.
Take a few minutes every run to evaluate your form. Think about your center of gravity. You want your chest slightly forward from your pelvis with your foot plant slightly in front of your center of gravity( see explanation). If you are leaning too far, you will feel like you are falling, and if you are tipped back your foot will be breaking your stride a bit and can lead to lower leg injures.
Focus on relaxing your upper body. Starting with your jaw, make sure you are relaxed and loose. Next, think about your shoulders, again relaxed, back and down. You will get a better arm swing and hopefully open up your stride a bit. Remember to keep your wrists and hands relaxed as well.
I like to go through this quick checklist while I am doing easy runs and even workouts to make sure I have as little tension in my body as possible. It seems to translate into a relaxed, comfortable stride in my experience. I also find that at the end of a race when I am starting to get tired or stressed, really focusing on my form helps me feel like I am getting a second wind and calms my mind.