You can try a pen with a thicker body. Sometimes if a pen is too thin, you find yourself squeezing harder with your fingers.
Also, some pens/pencils require you to press hard to write, which might contribute to finger/hand fatigue... You might try switching to pens that write with less pressure, like the Pentel Uni-ball pens.
And last but not least, make sure you periodically stretch your hand, wrist and fingers to their maximum range of motion to prevent repetitive stress injury.
HTH!