Not having a specific goal.
Without a specific detailed goal statement you're not going to lose weight as quickly and efficiently as you could.
Not creating and building on powerful momentum from the start.
Momentum is everything when it comes to fast and healthy fat loss.
Not taking steps to increase energy levels.
High energy is a must if you want to be able to make the all-out effort required to lose fat quickly.
Letting yourself get hungry.
Hunger has no place in an effective fast weight loss program.
Not avoiding simple sugar and refined flours and fats.
If you eat simple sugars, refined flours, or trans fats you have little chance of achieving fast, healthy weight loss results!
Not making lean proteins, good carbs, and healthy fats the cornerstones of your diet.
For best weight loss results, 90% of your calories should come from lean meats/proteins, "good" carbs, and healthy fats.
Not learning how to rotate/cycle your intake of starchy carbohydrates.
Few people can completely avoid starchy carbs (breads, cereals, potatoes, rice, etc.). Learn to rotate/cycle/shift your starchy carb intake and you'll be able to enjoy these high-energy foods while still maintaining a "hot" fat-burning metabolism.
Not doing both strength training and cardio exercise regularly.
Exercise is 100% necessary to burn calories and fat as quickly as possible. Don't buy into the marketing BS/hype surrounding many diet programs and "fat burner" supplements that promise fast, easy fat loss with no exercise. You'll need to perform both strength exercise as well as cardio exercise in order to build a leaner, fitter body quickly.
Taking diet/weight loss supplements that provide little or no benefits.
Most diet supplements are total crap. Focus on the few supplements iwht proven weight loss/health-boosting benefits: whey protein, multivitamins, and omega-3 fats.