1. Less Repetitions, Higher Weight. For years the myth of 'less repetitions and more weight equals size while more repetitions and less weight equals muscle tone' has been spread to everyone. There is a difference between a high and low repetition workout, but it isn't the way you'll end up looking. If you focus on doing high weights with low reps, your muscle strength will increase. If you focus on doing on doing a lower weight with more repetitions, your muscular endurance will increase. Muscle strength will give you the ability to pick up heavy objects. If you try keeping those objects held up, your muscles will fail pretty quickly. Muscular endurance will give you the ability to pick up lighter objects and keep them in the air for a long period of time (this is a simple explanation and in real life, the difference isn't so black and white). If you're trying to add a lot of muscle, the key is eating and lifting a lot. If you're trying to tone up, you need to go on a calorie restricted diet and do a lot of cardio to burn that layer of fat that is covering your muscles. Muscle tone has very little to do with the amount of weight you lift and the repetitions you complete.
2. More Exercise = More Results. A lot of people think that if you do more exercise, your results will be better and come quicker. More exercise is only beneficial up to a certain point. After that, the gains you made can start to reverse. When you exercise, what you're really doing is creating small tears in your muscles that will eventually heal stronger than they were before. If you don't give your body the rest it needs to make these repairs, you'll keep tearing away at your muscle fibers and never make any progress. More exercise is good, but taking it to an extreme isn't. Avoid working out more than 5 times per week.
3. No Pain No Gain. 'No pain no gain' is another myth that has been perpetuated over the years. The pain refers to working hard.
You do need to work hard to see results but not to the point of vomiting or injuring yourself.
4. Sit Ups. If you're looking to get a nice set of six pack abs, working them out every single day won't get you there any quicker. Having an ab workout is fine but when you take it to an extreme (read #2) your results won't appear any faster. Most of the time, the reasons why you can't see your abs is because of a high body fat percentage. The only way to get rid of body fat is through a calorie restricted diet and cardio. Spot reducing, getting rid of body fat from a certain area of your body, isn't possible.
5. Women Shouldn't Lift Weights. A lot of women are afraid to pick up strength training because they don't want to get as big and muscular as a lot of guys in the gym are. Because of this, most women stick to doing hours of cardio per week with no strength training. Cardio has some very positive health benefits but so does strength training. You can't be healthy without having both elements in your routine. The truth is that women do not have the necessary hormones to get as big as guys. As long as you're not taking any questionable substances, strength training won't make you big and bulky.