All too often people simply try to lift too much weight. By doing this you put more pressure on the shoulders and reduce the amount of effort on the biceps. Leaning back while lifting can also lead to back injury. Ensure you always lift within your comfort zone, keep your back straight and work from the biceps.
Crunches should work your abs, so they need to be controlled and use your core. When you tuck your chin in you strain your neck. If you are straining the neck, then you are making no difference other than potentially injuring yourself.
Not keeping the shoulders and back down is one of the most common problems with a chest press. This reduces the work on the chest and increases the chance of injuring your shoulders. If you can’t maintain your form then the likelihood is that you are lifting too much weight. Ensure you keep the shoulders and back down and contract your abs throughout the exercise.
The plank is a brilliant exercise for developing your core muscles, however many people sag at the hips or raise the bottom too high. By doing either of these things you reduce the effectiveness of the exercise, and by putting pressure on the lower back you may end up with pain. Ensure you keep your legs straight and hips up to create straight line from head to toe.
Poor technique with a lunge can put stresses on your knees and your neck. The most common error is for the front knee to lean over the toe, putting pressure on your knee. Leaning the upper body creates a strain on the neck. When you lunge, step forward and lower your hips until both knees are bent at 90 degrees. Keep your body upright and look straight ahead.
I probably do all of these mistakes if I'm not concentrating properly. That's the key - focusing on proper form rather than just counting the number of reps.
Dropping my hips during planks is easy to do, so I do focus on keeping straight, though bizarrely, this tends to hurt my back more.
I've never forgotten the neck strain from getting my crunches wrong - that's not a mistake you make twice. Thanks for helping others avoid it in the first place.