The key to continually improving any part of your fitness capabilities, you must learn how to progressively overload your muscles and provide new stimulus for growth!   

Our bodies are super adaptive to what we continuously do, and once it adapts, it gets used to what you're doing and will stay the same unless it's FORCED to change. If you just keep doing the same exercise for the same amount of reps, with the same amount of weight, you're going to get used to it and stay the same!  

For example, if you do 4 sets of 10 on the flat bench press with 135 lbs on a barbell, then you do that every Tuesday at about the same time for the entire year. A couple months in at the most and your body will already be adapted to this exercise. Now, after a couple months, if you decided to do 4 sets of 10 on the flat bench press with 145 lbs on both Tuesday and Friday, then congratulations!! You just created a brand new stimulus and now your muscles are firing on all cylinders, knowing they have a bigger resistance and need to make some adjustments to lift that much weight for that amount.   

This is the process of progressively overloading your muscles. Once you hit your goal and are able to complete the new stimulus without your muscles failing on you, we need to rinse and repeat. Continually apply this process with any type of physical activity and you will keep making progress!