"Theoretically, it'll work fine. Practically, it won't work and is going to be a big fucking mess."
-Andrew Lewis during a design meeting
In engineering and life, the word "theoretically" is thrown around a lot to explain that something CAN physically be done. The sentence is usually followed by "practically" along with an explanation of why the thing that CAN be done WON'T work. This dichotomy is exemplified in fat loss. Theoretically, losing fat is extremely easy. The right diet and exercise plan will invariably lead to the goal with minimal effort. Honestly, it's really not that hard... theoretically. Practically, a lot of obstacles get in the way and subvert the plan.
The first barrier that gets in the way is not setting SMART goals: specific, measurable, accountable, realistic, and time-based. When more of these criteria are applied when defining the goal, the greater the likelihood of success.
"I'm overweight, so I'm going to eat less until I lose some weight."
This is an example of a poor goal setting. There is clearly a problem (being overweight). There is a goal (lose weight), and there is a method (eat less). However, all three of these are not well defined, and what will likely happen is that the subject will not achieve their goal and become fed up, disheartened, and frankly, pissed the fuck off that they're not losing weight.
"I weigh 250lb and want to lose 30lb. I'll lose 1lb per week on average and my goal will be to weigh 220lb in 8 months. To do this, I'll eat 200 calories/day under my maintenance level. I will measure my food intake and use MyFitnessPal to monitor my caloric intake. To measure my weight, I'll weigh myself every morning and record the value to see my daily average of the week."
This is far more specific and will be more likely to result in success. If progress is not being made, it will be readily apparent and alterations can be made from there. The most important metric for fat loss is measurement: if you are not measuring your success is some way, you will not make progress as effectively or efficiently. For fat loss, I recommend three metrics: bodyweight, pictures, and waist measurements (at the belly button, relaxed). However, any metric can be used as long as it tracks progress. A tee-shirt or pair of jeans you wear often will show similar progress as the waist measurement.
The next issue is making smart choices in the grocery store. Just like in the gym, you should know exactly what you're going to do in the grocery store before you get there. The most important aspect of shopping is that you need to have self-control at the grocery store so that you don't really need to have self-control at home. Don't impulse buy, and don't shop hungry. This also means that you plan out food choices that excite you. If you're excited about what you're going to eat at home or at work, you won't feel as tempted to get fast food or eat pizza that someone brought in for work. By extension, this means you need to learn how to cook: an important skill in its own right.
Making exceptions will also get you into trouble. There is a slippery slope when it comes to exceptions. You have one donut at work thinking it'll be ok. Next thing you know, you're drinking half a gallon of your wife's chocolate milk. I've been there. If you're one of these people, you must be uncompromising when you diet to lose fat. Nothing happens spontaneously. If someone offers you a donut at work, politely decline. However, if you know it's a coworker's birthday tomorrow and they'll bring in cake, you can plan on having some and build the rest of your diet for the day around that. The same is true of holidays. Enjoy the holidays, but don't go crazy. When you aren't uncompromising, you'll always find a reason why you can make an exception. Holidays, birthday, and celebrations occur more often than you'd think.
The last piece of advice regarding what can go wrong in your diet will pervade the rest: your supporting environment. If you have a spouse who has the same diet base as you, gives you the "Hey, don't eat that. Be strong", and helps you prepare your food on Sunday, you will have a much easier time losing fat. Conversely, a spouse, co-worker, or parent who brings you down (deliberately or otherwise), will make the whole process very difficult. You will find out a lot about your friends, coworkers, and spouse when you try to stick to a diet. This may sound dramatic, but it's true. If it becomes an issue, let your friends and family know that you are trying to better yourself and even though they're trying to "just be nice", they aren't helping.
All of this may seem obvious, but when you first decide to go on a fat loss diet, you may not consider these obstacles until they get in your way. Have a supportive environment. Don't compromise until you know you can handle it. Have a plan before you go to the grocery store, and buy exciting food. Finally, use well-defined, measurable goals to design your fat loss program and use metrics to track your progress. Theoretically, fat loss is exceedingly easy. Practically, it can be crushingly difficult.