When doing sit-ups, it is important to protect your neck. If you don't, you can suffer a serious neck strain. When you are doing sit-ups, put your tongue on the roof of your mouth. That will help keep your head properly aligned and prevent any muscles strains in your neck.
When you get injured, it is important that you continue to exercising, though make sure to not push your injured muscles. By excising the injured area slowly and carefully you will promote healing. By gently stretching muscles that are injured, you increase the flow of blood and oxygen to them.
When you are doing arm curls, it is recommended to flex your triceps after each repetition. You can do this by completely straightening your arms. It is important to flex your triceps each time because it ensures that each muscle is being worked on through its entire range of motion.