Do not try to hit "hard" when you punch. You will tense your shoulders and back, and exhaust yourself quickly. Hit with relaxed speed using body snap. People who hit with tension tend to punch with just their arms, which do not have much mass. Use the entire body to punch. Tense fighters also tend to telegraph their punches.
In Olympic style sparring, you must move your opponent to score a point. The head is easier to move than the body, so head kicks are more effective than body kicks. They are also more noticeable to the corner judges. However, your kicks must be controlled and the contact has to be light or you might be penalized, especially if you are a color belt. In no-contact sparring, you receive more points for kicks to the head but you must exercise good control since excessive contract will result in a penalty.
In Olympic style sparring, do not so caught up in whether or not you get touched that you miss opportunities. Take a weak shot to the abdomen and counter with a knockout shot to the head. Never throw weak shots yourself.