A lot of people get to their ball in a fairway, measure the flag and choose their club. But this is not enough information. Instead of measuring the flag and guessing the other yardages, be more accurate. Measure bunkers, bushes and, if possible, any drop offs or slopes.
Use this information to help you to make a decision on the shot you would like to hit. For example, if the distance to the front of the green is close to the distance to the flag, make sure the shot will go at least far enough to get over onto the green.
The more data you have on the golf course, the more you can make accurate decisions and prevent avoidable errors. Once you have all the information available, you will be ready to execute the shot with much more confidence and accuracy. This will allow you to prevent mistakes and capitalise on opportunities.