Big scores are the backbone around which an innings can be built.
Physical fitness is very important if you are to play a long innings. Cricket is often played in hot weather, and come the end of a long stint at the crease, you will have done a large number of short sprints up and down the pitch, all while wearing gloves, pads and possibly a helmet.
Concentration is vitally important. It is virtually impossible to concentrate throughout a full session of play, so learn to relax at certain moments, such as when fast bowlers are walking back to their mark, or at the end of an over. But when you are preparing to receive a delivery, total concentration is paramount. Watch the ball all the way. Don't let external worries trouble you - learn to block out the sledging of the fielders, and if you play a bad shot but get away with it, concentrate on doing the right thing next ball.
A good coach can teach you how to execute each of the common strokes, but the best coaches can help you with when to use each of these shots. Learn to judge line and length early to help you decide which shot to play, when to defend and when to leave the ball. The pitch conditions could also affect your shot selection, e.g. the pull stroke may not be advisable if the bounce is low. Many a batsman has met their downfall by selecting the wrong shot for a particular delivery.