Think like a Hacker

No software is 100% secure. The hackers of the world know this and will seek to find vulnerabilities in your software and exploit them. As a tester, you'll need to be devious and conniving. Try to think of what things of value your software contains, why someone might want to gain access to them, and what the methods of entry might be for them to get in. Don't be gentle. They won't be.