What causes cramping? What can I do to avoid cramping?

Cramps. When I workout, I can squeeze pretty much any major muscle into a cramp. My workout style is that of moderate weight, hold and squeeze, until it burns like mad... and go at it HARD (i.e. sweaty mess). This works great for me in terms of gains, albeit when I don't cramp up... but lately it's been getting worse. I'm wondering if there is something lacking in my diet or style that needs to be altered. Thanks!