in the past, I would squeez as hard as i could for as long as i could, until i got mega muscle burn. Do it three times, and on the third one when you cant hold it any longer hold it for ten more seconds. It will be exhuasted! that is good!
I would do this excersize every other day to give it rest. do it for a month and it will get soo hard! do it for two months and it will be a steel pipe, three months...... I dont even know!.........i didnt do it that long, haha
BxB