A common mistake in exercise especially with weights is always increasing the amount weight you lift to keep things effective. The next most common mistake is always trying to do more exercise to try and get more results. There are smarter and far more effective tweaks and tricks you can use to dramatically increase the effect without grinding your gears.
Have a look here at a couple of subtle but killer adjustments which spice things up big time without making the moves dangerous and without increased performance sloppiness from balancing too much load and/or grinding out reps while too fatigued.
Includes audio commentary.
These examples are applied to shoulders but can be adapted for almost any exercise choice.