#1: Sticking to Cardio Only
Women especially tend to camp out in the cardio section for their whole workout. It may the fear of bulk (which we all know now is crazy talk) or it may just be because you prefer cardio, but cardio is only half the picture. It is true that if you are trying to lose weight, cardio should be your number one focus, but without building muscles, you will still be flabby- you’ll just weigh less. (trainers call these girls "skinny fat") Muscle also boosts metabolism, so the more muscle you have, the more calories you burn through out the day, even at rest. You should strength train a minimum of 2-3 times per week, allowing at least 24 hours of rest in between workouts so your muscles can repair themselves.
#2 Always Using Machines
Machines serve their purpose: they’re pretty self-explanatory, and, well, that’s about it! If you are trying to learn the form of an exercise, machines are a great way to get used to the proper feeling. However, they only work one muscle group at a time, and since they keep you in proper form, your stabilizing muscles don’t get a chance to work along with the primary muscle. The problem with that is when you go to use that muscle in real life, the other muscles that help it out aren’t as strong, therefore hindering your performance. Free weights allow you to multitask, making for a shorter workout (squats with bicep curls) allows you to burn more calories. Training your body in the way it is designed to move, through free space, as with dumbbells and cable machines, is going to use more muscles, and produce greater results.
#3 Not Having a Plan
Time is very important to everyone and I’m always overhearing people talking about the lack of time they have. An easy way to avoid wasting time and energy during workouts is to come with plan. Write it down or mentally come prepared so you can quickly move from one exercise to the next without meandering and make sure you hit every muscle group instead of winging it. This also helps if you’re at the gym during busy hours, you can easily move on to your next exercise without hovering. A personal trainer is a great tool to jump-start your program. They’ll create a customized & progressive plan for you based on your goals and may show you exercises you haven’t thought of.
#4 Doing the Same Routine Over and Over Again
A plan is important, but switching it up is paramount. Your body is smart, but also lazy! If it can figure out what you are going to have it do, because you’ve had it do the same routine for the past 3 months, its going to figure out how to do it with the least amount of energy possible: this is called a plateau and can be very frustrating. I’m sure you or someone you know has experienced this, “I’ve been going to the gym 6x a week now for 3 months now and my body hasn’t changed!” Switching up your routine every 3-4 weeks will keep your body guessing and keep the results rolling. Changes can be in the actual exercises or equipment or as simple as weight, sets or reps. It’s best to change 1 or 2 variables!
Hope this helps :)
After being a personal trainer at Equinox in Santa Monica and working as a yoga teacher at several studios including a gym, I've seen it all. Students wobble into class injured, saying they can hardly walk due to an injury from their last class. I've had students not come to my class for months because of an injury they received after going to another yoga class that took them far beyond their capabilities. Obviously, this is all subjective and students/clients should monitor their own bodies. But seriously, some teachers are out of control. I've heard about a certain incredibly famous power yoga teacher actually popping a students rib out of place from pressing too hard on his back. Some teachers assist and have no proper training in how to do so. Not cool.
Same goes for cardio and especially for weight lifting. I had many clients who insisted on benching more weight "because it's what they did in college" but what they don't realize, is that they aren't that person anymore. That's when they get injured and then have to lay off exercise for weeks to properly heal. 
Part of the problem is that increasingly, the people teaching yoga don't know enough about it. Yoga was traditionally taught one-on-one, over many years. Today's instructors can take a yoga teacher training course in just one weekend. Luckily, to be in the 
After being a personal trainer at Equinox in Santa Monica and working as a yoga teacher at several studios including a gym, I've seen it all. Students wobble into class injured, saying they can hardly walk due to an injury from their last class. I've had students not come to my class for months because of an injury they received after going to another yoga class that took them far beyond their capabilities. Obviously, this is all subjective and students/clients should monitor their own bodies. But seriously, some teachers are out of control. I've heard about a certain incredibly famous power yoga teacher actually popping a students rib out of place from pressing too hard on his back. Some teachers assist and have no proper training in how to do so. Not cool.
Same goes for cardio and especially for weight lifting. I had many clients who insisted on benching more weight "because it's what they did in college" but what they don't realize, is that they aren't that person anymore. That's when they get injured and then have to lay off exercise for weeks to properly heal. 
Part of the problem is that increasingly, the people teaching yoga don't know enough about it. Yoga was traditionally taught one-on-one, over many years. Today's instructors can take a yoga teacher training course in just one weekend. Luckily, to be in the