I've had luck with the Grip Master (and other derivatives) series of hand exercisers. I can easily and quietly use it at my desk at work while mindlessly working on something else.

There's a PDF that Prohands (the originators of these products) that has many exercises available for musicians with demonstrations; but I've found, specifically with guitar the fingertip/small of thumb grip (They called it Finger Tip Pinch) has worked best for me.

It does increase strength, but stamina - as mentioned above - is also a large factor. So doing reps while holding the grip down for 3 to 5 seconds at a time is really where all the benefits come from.

Here's a quick demonstration of my favorite grip.

Another benefit to these is the warm-up time when playing is drastically reduced when used lightly beforehand.

However, with any sort of exercise, too much of a good thing is bad, so trust your hands, do what feels right and think about the motions, grips and movements you make on the guitar; for repetition is key to a musical instrument.

I've had luck with the Grip Master (and other derivatives) series of hand exercisers. I can easily and quietly use it at my desk at work while mindlessly working on something else.

There's a PDF that Prohands (the originators of these products) that has many exercises available for musicians with demonstrations; but I've found, specifically with guitar the fingertip/small of thumb grip (They called it Finger Tip Pinch) has worked best for me.

It does increase strength, but stamina - as mentioned above - is also a large factor. So doing reps while holding the grip down for 3 to 5 seconds at a time is really where all the benefits come from.

Here's a quick demonstration of my favorite grip.

Another benefit to these is the warm-up time when playing is drastically reduced when used lightly beforehand.

However, with any sort of exercise, too much of a good thing is bad, so trust your hands, do what feels right and think about the motions, grips and movements you make on the guitar; for repetition is key to a musical instrument. Also, go with quality over quantity. Don't be like me and spring for the heaviest tension exerciser they have...