getting infront of a mirror is important if you can't work with a trainer. There should be a straight line from shoulder to your heel (obviously angled). The biggest mistake is dropping the hips too much which puts a ton of stress on the lower back. Keep your palms squared under your shoulders and keep your neck relaxed. It is better to hold it for a shorter amount of time using proper form than a long amount of time with incorrect form.

I love plank and side plank.