One of the biggest mistakes, that still occurs daily, is the misconception that crunches alone will shrink your belly. Walk into any gym and I can almost guarantee you'll spot a middle-aged man or women performing crunches with poor technique. The problem is, this very rarely addresses the fundamental problem. You see, we all have abs, the reason we can't see them though is because of a layer of fat covering them up, and while crunches do activate the abdominal muscles, they do very little to help burn off this fatty layer.
The answer to this is to use compound movements. You can think of these as 'big' movements which use multiple muscle groups which, in turn, uses up more energy in the form of calories. These types of exercises help accelerate results and provide a more efficient workout for your time invested. These specific exercises also have a positive effect on your metabolism, leaving it running in a higher gear after you finish, helping you burn yet more fat. To get all of these effects, I strongly recommend you include exercises such as that require you to pull, press and squat as much as possible.
Another way to accelerate your fat loss efforts is to use interval training. This isn't the same as jumping on a treadmill and jogging consistently for 20 minutes or so. Interval training requires you to alternate between high and low speeds for the full duration. An ideal work/rest ratio would be 20 seconds high intensity work followed by then 40 seconds low intensity slow jog/walk repeated 20 times (20 minutes total). Research shows you can burn up to 3 times more fat compared to steady state exercise of the same duration.
A few other general gym rules include keeping your workout as short, sharp and intense as possible and having a predetermined plan heading into the gym to avoid time wasting. If you stick to everything discussed above, you will no longer need to worry about crunches. Of course, work in the gym is only half the equation. The remaining work is done in the kitchen and is concerned with what you eat and when.
In general, a healthy, well-balanced diet will serve you well. One that includes lean sources of protein (eggs, meat, fish, dairy, legumes), complex carbohydrates (pasta, rice, lentils, bread), healthy fats (avocado, nuts, olive and flax oil) and plenty of fresh fruit and vegetables will provide everything your body needs to produce energy, burn fat and keep you full. Also, don't forget to manage your fluid intake. Keep a water bottle with you during the day and limit your intake of soda's and alcohol to just 1-2 a week. Also, depending on your needs, you may wish to invest in a whey protein powder to boost your protein intake. You can mix these in a blender with frozen fruit and low-fat milk. Pea protein and other vegetarian options are also good if you can't tolerate animal-based proteins.
If you follow all of the principles outline here you can expect to see results within 4-6 weeks. The most important factor in your success is going to be your level of consistency which is why I recommend you keep a training diary to monitor your progress and help you make changes when necessary. If you have any further questions, please leave them in the comments section below.