I've found that if you change how you wrap the yarn around your "throwing finger" it changes your tension. For instance, I usually wrap the yarn around my index finger, but if I want the tension to be really loose I'll wrap it around my middle finger. Or really tight, I'll use my thumb.
But I agree that it's really hard to change your tension on purpose. The only thing I ever use it for is tightly knitting ribbing on socks for my wiggly-toed, hole-inducing husband. (It lasts longer.)