The Braidy Tester blog that I just started reading really has a very nice summation of the mentality of a tester.
More than just breaking software, the tester should make sure it has the capabilities the customer needs.
Too often, I either work with employees who are testers but not really focussed on the end product. Rather they test the technology. Thats not enough by itself … the testing phase should go ahead into phase 2 because customers today have so many choices and can compare your products to other better designed ones. You constantly need to give them a product better than the competition, not just better than your own previous version.