"Proper running form" is much more comprehensive than just the foot strike. Running with a fore/mid/heel strike are all valid IF you're not suffering as a result and the rest of your mechanics are sound. Focusing on foot strike alone will not yield a more efficient run. It is my opinion (after several years of running and trial and error) that getting your body mechanics in line is the most important focus. Your foot strike will naturally evolve into what works best for you. Here is a good article (if slightly dated) on The Science of Sport that talks about foot strike.
Your head is key to your overall posture and that determines how efficiently you run. Look ahead naturally, not down at your feet, to straighten your neck and back and bring them into alignment.
Shoulders play an important role in keeping your upper body relaxed while you run. For optimum performance, your shoulders should be low and loose, not high and tight.
Your hands control the tension in your upper body, while your arm swing works in conjunction with your leg stride to drive you forward. Keep your hands in an unclenched fist, with your fingers lightly touching your palms. Your arms should swing mostly forward and back, not across your body,between waist and lower-chest level. Your elbows should be bent at about a 90-degree angle.
Holding your back straight allows you to run in an efficient, upright position that promotes optimal lung capacity and stride length.
Your hips are your center of gravity and when combined with proper torso position they keep you pointing straight ahead. Tilting your pelvis can put pressure on your lower back and throw the rest of your lower body out of alignment.
Efficient endurance running requires just a slight knee lift, a quick leg turnover, and a short stride. Together, these will facilitate fluid forward movement instead of diverting (and wasting) energy. When running with the proper stride length, your feet should land directly underneath your body. As your foot strikes the ground, your knee should be slightly flexed so that it can bend naturally on impact. If your lower leg (below the knee) extends out in front of your body, your stride is too long.
Here are some articles on general proper running form that will provide addiitonal insight: