I know this thread is old but a big part of the diet cycle is understanding glycemic impact. When you eat simple carbs like white bread, potatoes, white rice, candy etc, even in small portions, the sugar goes into your blood stream like a large bolus dose. This causes a spike in insulin levels which quickly works to transport the free sugar from your blood to your glycogen reserves (Which fill up quickly) and the balance gets stored in your fat cells. This insulin spike is followed by a "crash" in blood sugar levels which triggers cravings for more carbs. It's really these cravings that undermine everyone's diet.
If you switch to complex carbs you will avoid the insulin spikes and blood sugar crashes and curb your carb cravings. This is also why the Atkins diet works for most folks because by eliminating most/all carbs you can minimize cravings and you reduce glycogen reserves so you go into fat burning mode very quickly.