Opportunities comes but once in a lifetime. That is what a wise men will tell you. It means that you should be able spot an opportunity and be brave enough to grab it when it is presented to you. However, many, me included, failes to take the opportunity when we see one simply becuase we were afraid that we might make the wrong judgement. Those who are brave enough to try are those who made it through.
Sometimes it is not because we are afraid to try, but most of the time, we fail to spot it even if it is laid right in front of us. A good opportunity is hard to spot. Things that seems so simple to us can be a money churner for the guy who sees the potential. It is only when others have succeeded that we regretted not doing it in the first place. That is normally how we let an opportunity slip by.
Having said that, there are also those who sees an opportunity in everything. These opportunists are willing to give everything a shot. He might land himself a big one one day, or he might just waste away his life trying to do too many different things in his life.
It is hard. Whether you grab yourself that big one or whether you let it slip by will depend much on your luck as well as your foresight. This is something that you will never find in any books that you read. No one book will tell you how you can spot a good opportunity.