As the old saying goes "variety is the spice of life", it's also the spice of your workouts. Adding variety to your sessions is a simple way to keep your body guessing and progressing.
If you just think variety in your workouts would be just changing your reps and sets you'd be wrong! Check out this list I've put together that includes variables I, and my clients regularly change in our workouts to keep the results coming...
Number of Sets
Total workout duration
Grip or stance width
Grip diameter (thick bars etc.)
I'm sure there's many more, but the variables above are enough to keep your workouts fresh. My personal recommendation is to perform a workout no more than 6 times, or less if you adapt to it faster...