1. Not accounting for all calories. It's easy to do... a handful of nuts here, a nibble of a friend's pasty at the office, a couple pieces of chocolate when you get home... they all seem pretty innocent, but all of those little bites could be adding up to HUNDREDS of extra calories a day. That can make a BIG difference in any changes you are seeing (or NOT seeing) in your body. How can you fix it? Keep a food journal! If you have to write everything down, you'll be more likely to pass on all of those little nibles. My favorite place to keep a journal where I can track my weight and calorie goals is with thedailyplate.com. I know other people use myfitnesspal.com too, just go with whatever works for you and is convenient! Once you get started, you'll see results and want to keep up with it.
2. Depriving yourself. I've seen it time and time again. A friend goes on the HCG diet where it's so low in calories (500 a day!!) for 30 days and sees results on the scale. And then gains it all back within a week. This is not the answer! Repeat after me: I am making a LIFESTYLE CHANGE! Any kind of diet that is going to have you eat so few calories that you are STARVING is not a good idea. You shouldn't be hungry all the time, you should be satisfied.
3. Relying only on diet to lose weight. Again, this may work in the short term, but if you want to be healthy in the long run you need to exercise. Muscle burns fat, so make sure you are doing a combination of strength training and cardio to keep your heart healthy and your metabolism on fire.
4. Skipping meals. So you're going out to eat at a fancy restaurant and want to save all of your calories of the day for the big meal. You eat nothing all day, and then you are so hungry that you eat everything in sight at the restaurant. Sorry, this is no good! Not only are you probably consuming more calories in that one meal than you would've if you've just eaten small, healthy meals throughout the day, but you've slowed down your metabolism in the process, and turned yourself into a grouch. Seriously, it's no fun to be hungry! If you plan on going out to eat, just be conscious of what you have that day and make sure to write it down in your food journal.
5. Thinking health foods are calorie free. I'm guilty of this as well! I went through a time in college where I was eating so so healthy yet I still couldn't get down to the weight I wanted. I was overdoing it on healthy foods like fruit and nuts. These foods are great, don't get me wrong, but they aren't calorie free! Make sure to account for EVERYTHING that you eat in that food journal. That way if you're not seeing the progress you desire, you can look back and see what's going on and what you can change.