There are 3 significant parts to losing weight and most people use 1 or 2 parts but neglect the 3rd and do 1 and 2 incorrectly.  Here are the 3 parts and how they are meant to work.

1.  Diet!

This is the most popular of the 3 parts but it is almost always misunderstood.  Your diet should reflect your exercise plan.  It is important that you have a balanced diet, not the no carb fad diet.  
The reason for this is simple, if you don't eats carbs, you don't have energy, if you don't have energy, you don't exercise!  Another thing that is misunderstood is fat.  You need fat in your diet.  I'm not talking about going to McDonalds and picking up a Big Mac everyday.  The right fat to eat is Omega-3.  This fat is found in nuts, olive oil, fish, etc.  Omega-3 is important in building your heart health which is directly linked to weight loss.

2.  Exercise!

An important part of exercising is to "shock" your muscles.  I call this muscle confusion.  The way it works is, when you do the same workout everyday, you will reach a plateau in which you won't lose anymore weight.  Once you reach this plateau, change your workout in order to work your muscles in a little bit different motion.  For example, say you normally do the bench press and you reach this plateau.  Switch from the bench press and start doing butterflies and you will begin to see results once again.  If you plateau again, simply switch back to bench press.  

3.  Sleep!

This is the most ignored part of weight loss.  Sleep is directly linked with weight loss.  Studies have shown that if you get at least 7 hours of sleep every night, you are more likely to eat less. Less than 7 hours of sleep increases your appetite hormone and you are much more likely to skip the workout if you are too tired.