I can't believe how many times I hear someone say don't set goals, you are setting yourself up for failure, my first thought is always WTF!? Not having set goals is like taking to the high seas in a schooner without a rudder. I hear them say "envision" Well if you are sailing in a schooner with no rudder you can "envision" the beaches of Tahiti until the cows come home but I seriously doubt you will ever arrive! Unless you have a charted course and a way to check your progress along that course, you'll end up like the Flying Dutchman, aimlessly sailing around the seven seas in utter confusion.
The key is in properly setting your goals. No you will not reach every goal you set. Goals should be challenging but not so difficult as to be improbable. If you set sail in a schooner, having a goal of reaching the moon is rather asisnine. But people do it everyday. They get into a business and set a totally unreachable goal.
The first thing you need to do is to establish your long range goal. Where do you want to be 5 or 10 years from now? What will it take to get you there? Once you establish that little fact you can set your goals accordingly. Remember this is a long range goal not an over night goal. The next step is to figure out your yearly goals, your Quarterly goals and your monthly goals. Why do you break them down like this? Remember I said you will not reach every goal but sometimes you will go over the goal you set.
Lets say your goal is $500 per month, you first month you make $300, the next month you do really well and make $1000 and then the 3rd month you make $450 that's a grand total of $1750 for the quarter. Even though you missed your goal two of the three months you are still on track to reach your annual goal since you not only met but exceeded your quarterly goal. Theres another reason you set quarterly goals, they give you a checkpoint to see if your are on track or need to make an adjustment.
To go back to that sailing analogy; tides, currents and winds can take you off course. Maybe not by much but it doesn't take much deviation to put you way off course if you allow that deviation to continue. By periodically checking your heading, you can make minor adjustments today that will correct your course rather than having to plot a whole new course when you are hundreds of miles out of your way.
Understand that goal setting is vital if you want to be successful. But you must set reasonable goals and monitor your progress for it to be effective.