Breaking up large weight loss goals into smaller increments can be a great motivator. If you want to lose 50 pounds then you could break that into easier but smaller goals of a few pounds each. This makes it easy to see the progress you have made. Don't forget to celebrate your mini-goals as you attain them.
Be realistic with your weight loss goals. Like with any other project, if your goals are unattainable, then you have no chance of success. Even if you are trying to just drop 20 pounds in too short a period of time, you're just setting yourself up for failure. Instead of setting such an unrealistic goal, give yourself more time and set a goal that you may be able to attain for that specific week. Keep the big picture in mind, but don't focus on it. Just focus on one week at a time instead.