Also, you'll want to call on the outer third of the call, not on the rim but about an inch in or so, the closer you get to the rim the sqeaker it will sound......

If your turning your skin white holding the striker, your holding it a little tight. Loosen your grip, you need only hold it firm enough to control it and allow different down pressures until you feel you've got the sound you need. It doesn't matter what call, glass, slate, alum or what not. Hold your striker straight up, slightly tilt it away from yourself and make small ovals...alot of people with go back and forth and not get the whole range of the yelp....small ovals or footballs will assist you in getting the yyyeeelllppp instead of a eeeerrrr, eeeerrr sound....