The key to improving your results in the gym is to increase intensity every workout.
If you do not increase your workout intensity your body will never be challenged and any results you might have gotten might even reverse.
Examples of increasing intensity would be increasing the weight, increasing the number of reps, or increasing the number of sets in each workout.
Your muscles get bigger when they are under stress.
The more stress you can put your muscles under, the better your results.
Increasing any of the previously mentioned items will bring you results for a while.
The problem is that your body plateaus if you stick with the same exercise and same way to increase intensity.
This means that you no longer can challenge your body and you are stuck on the same weight or reps.
This is not good and you want to avoid this.
In order to avoid plateauing you must change the TYPE of increased resistance that you use every workout.
A proven way of doing this is to define a rep range for your goal.
This could be 6-8 reps for strength or 8-12 reps for building muscle.
Then you start with a weight that will get you to the minimum number of reps that you have defined.
In the next workout you try to get one or two more reps.
Once you reach your max reps you increase the weight and go back down to your minimum reps.
An example of this would be the bench press.
Your goal is to build muscle so your rep range is 8-12.
You can bench press 150 lbs for 8 reps until you reach failure.
On your next workout you press 150lbs for 10 reps.
Then you get 150 lbs for 12 reps.
The next time you work out you will increase the weight to 160 lbs (or more if you can) and try to get 8 reps.
This is a proven method for avoiding plateaus and can be implemented in any workout routine for huge results.