There is a tendency for people to relax once they have reached a good position or to give up hope if their position is very bad. These attitudes are natural, but both lead to bad results.
Many players, even world championshave achieved winning positions, only to lose because they relaxed too soon. Even the best position won't win by itself; you have to give it some help! In almost any position, the "losing" player will still be able to make threats. The "winning" player has to be alert enough to prevent these positions.
Advice: If you have a better position, watch out! One careless move could throw away your hard-won advantage. Even as you're carrying out your winning plans, you must watch out for your opponent's threats.
Conversely, if you have a worse position, don't give up! Keep making strong moves, and try to complicate the position as much as possible. If your opponent slips, you may get the chance to make a comeback. Remember: Where there's life, there's hope.
So be alert all the time, no matter what the position is like. A little bit of extra care can pay off in a big way.