Have proper warm-up prior to practice or game. Make sure have extensive flexibility exercises after practice or game. Placing Ice on your legs after using ir is also a good way to stop any swelling.

Having proper leg strengthening will help also in this problem

http://www.softballperformance.com/catching-softballs-thankless-job/

Like Coach Lia said, it is really important to train for the various squats, not just show up at practice and games and go for it. Make sure you are using proper form for each of your squats. Where are you hurting? Do you use the knee savers on the bottom half of your guard straps, or on the upper straps? Some folks like them closer to the heels, and some like them right up in the crook behind the knee. Skim the below sites for catcher specific exercises and to recheck your stances. Then, make sure you train at least 4 time per week using catcher specific exercises/drills to strengthen yourself and hopefully get rid of the pain. Also, please check with a doctor to make sure there isn't more to it.

www.angelfire.com/sports/brookside/skillscathcer.htm
www.fastpitchlane.softballsuccess.com/categories/Catching.aspx
www.baseball-catcher.com/index.htm

he only thing i can think of is to stretch before the game and knee savers. i wish i could be of more help but im not a catcher im a pitcher, but my sister has been a catcher for a long time:) ill ask her sometime and see if she has any idea:)