If you have been working out for years, and are no longer getting sore, being challenged, and are no longer getting results, then your training may be stale. To avoid plateaus you must constantly vary your training. Randomizing the stimulus to your muscles will force them to constantly adapt and simply will not allow them get used to what they are doing.
You can certainly do different exercises to change up your routine, but you can also manipulate the total training volume, as well as the way in which you perform the exercise. On any given exercise you can control the intent, speed at which you travel, the angle of the working joint, and the weights and reps performed.
To change up the speed of a contraction you want to think about the 2 parts of any repetition, the up phase, and the down phase. Try going slower, and really feeling the contraction of each rep. The down phase should be slower than the up. For example try doing a 1 second up phase and a 4 second down phase. Also try a 3 second up phase and 3 second down phase. See how much earlier you feel the burn.
The angle of the working joints can be manipulated to change the intentions of an exercise. For example doing squats with more focus on pushing through the toes will put more tension on your quadriceps, and conversely if you squat into the heels you place emphasis on the glutes and hamstrings. This works really well for isolation exercises. Try doing bicep curls while sitting at 3 different angles on the bench and let the arms hang back. Try doing shoulder raises while leaning at different angles. Try a narrow push up then try a wide stance push up and feel the difference. The change of angle changes the resting length of the muscles being worked, which slightly changes the range of motion and the target area of the exercise.
The weights, reps, and sets of an exercise should change from set to set. Light weights allow you to do more reps, and heavier weights yield fewer reps. Next try doing a different weight/ # of reps in each set. For example start with 10 reps @70% of max weight, then on the next set try 6-8 reps of 85% of max weight. Once you peak in weight try breaking it back down by lowering the weights each set and aiming to burn out at each weight. This method is known as the pyramid. Super sets also are a great way to mix things up. A super set is any 2 exercises don’t back to back for the same muscle group. For example: maxing out pull ups and then jumping right into dumbbell rows to work the back. Your body will be struggling to keep up, and that’s the type of stimulus you need to keep progressing.
The overall take home message is to be sure to create enough of a challenge in your workouts. It should not be easy for your body to keep up with the demands you place on it. If it doesn’t challenge you, it will not change you!