Here is a list of mistakes to AVOID when strength training. By avoiding these mistakes, you will be able to improve your performance and take your fitness to new heights.
Only training upper body
Don't get too focused on how you look in the mirror. Having big arms and a broad chest may impress the girls and look good on the beach, but it won't do anything for your athletic performance. Your largest muscles are in your legs (including glutes and hamstrings), and the more you strengthen them the more hormones you release to help build strength and size in your entire body. Your legs are what provides your foundation to any sport-specific movement.
Always working on your strengths
It's easy to work on things you are good at, but that won't make you a better athlete. In order to improve you have to eliminate any weaknesses, and that means probably doing things you aren't familiar with or good at. Most elite athletes don't have weaknesses, and if they do, I'm sure they are working on them right now. When was the last time you did Deadlifts? They are one of the best compound moves (targeting several muscle groups) to help build strength.
Lifting weights too slowly in the gym
Do you always lift weights at the same speed? If so, you are missing a key component to improving your strength gains. According to Mike Boyle, of Mike Boyle Strength & Conditioning, "lifting weights with explosive speeds activates more fast-twitch muscle fibers, which have the greatest growth potential". You don't always have to lift weights with an explosive speed but it should be incorporated in your periodization training. For instance when lifting heavy weights (85%+ of 1RM) you want to lift slow and controlled. When performing movements with explosive speeds, do the lifting phase of each exercise as fast as possible.
Doing too much slow and steady running
Running too many slow miles is a repetitive movement and your body will adapt quickly and stop making progress. According to Boyle, "running doesn't activate fast-twitch muscle fibers, which are great fat burners and body builders". Start incorporating some intervals (short bursts of intense activity) in place of your slow runs, or run steep hills instead of always running on flat terrain. You can sequence your interval work many ways, for instance using a 1:1, 1:2, 1:4 or even a 2:1 ratio. Using the 1:2 ratio, this means that you would run hard for a length of time (say 30 sec.) and follow that with twice the amount of recovery time (which in this case would be 1 min.). According to a recent study in the Journal of Strength and Conditioning Research, interval training leads to gains in aerobic and anaerobic performance that are significantly greater than with steady state cardio.