My years of experience in many different types of gyms over the years have allowed me an inside look at some of the biggest mistakes many clients make when pursuing a healthier lifestyle. In all fairness to the client, there is so much information regurgitated and/or made up via the internet that no one knows what to believe anymore when it comes to training and nutrition.
Mistake #1 that I most commonly see is people not setting and pursuing attainable goals. Most just come in and train with arbitrary goals that lead them to frustration and then a quick fall off of the weight training wagon. The best way to combat this issues is to write down (with a book and pen) specific, testable goals, and most importantly, reassess often. This way you will keep your motivation high and the chances of success increase immediately. Whether your goal is to run a marathon, drop 10 pounds for the summer, or to finally get that chin up you've desperately been pursuing, write down your goals!
Mistake #2 that I most commonly see is people trying to out-train poor nutritional habits. The bottom line is that if you eat like a 5 year old (chicken fingers, french fries, snacks, etc), it doesn't matter how hard you train, you will not be reaping the full benefits of training. In-fact, not only will you put your muscle mass and bone density at risk, but you will also be increasing your risk of injuries in the gym because your body is being filled with calorically dense foods instead of nutrient dense foods. A "simple" fix is to start food journaling and write down everything that goes into your face every single day; you'll think twice about that Big Mac if you know you're going to have to write it down. Also, go back to the nutrition basics: Eat real foods that die and will grow mold if left out and unattended, like: (fruits/vegetables/meats/chicken/fish/potatoes/etc). The less processing your food goes through the healthier it will be and the better it is for you.
Mistake #3 that I that I most commonly see is people just not pushing themselves hard enough, safely in the gym. To actually change your body composition (gain muscle and lose fat) you have to move some serious weight. Now granted that's relative to each individual, but this is where a trained professional comes into play (strength coach or trainer). If you're spending 30 minutes a day on the hamster wheel, I mean treadmill and walking at a snails pace, and then you spend the next 30 minutes in the gym texting and taking selfies in the gym mirror, you are probably not seeing the results that you want to see. A simple solution is to hire a professional who will teach you the BASICS!! Forget all these silly, "Functional" bouncing and flipping around movements. Find a pro that will teach you how to Squat, Dead Lift, perform Chin Ups, Push Ups, Bench Press and Dumbbell Rows. If you don't have the means to hire someone, find solid information via youtube, good training blogs and dare I say, Facebook. Now I warn you, these avenues have become a haven for internet gurus who have little to no experience, and aren't good at anything but critiquing people who are actually training real live humans. If hiring a proper trainer isn't in your reach, teach yourself the basics, master them, and progress yourself properly. This will ensure that you stay strong, lean and injury free. But just remember this, you will pay for your health one way or another: You will pay a trainer and spend money on healthy food choices that you'll cook at home, or you will be paying for higher insurance premiums, doctor visits, prescription drugs and eating out, that all costs a lot too...