When I have a spare moment, I usually try and think about how to test something. In fact thats not true, what I do is actually test something. It might be an app on my phone, an online tool, parking-ticket machine or search engine. Usually it is what-ever is to hand, at the time. This is a good way to practice my skills, and can take as long as I have free. In fact having only moments is beneficial, you soon get better at finding out more issues - more quickly. For example, a few moments ago I thought I'd test Google's currency converter. If you haven't seen it, it looks like this: You enter a value and two currencies in the format shown, and Google will give you an answer with great precision. (I haven't examined the accuracy.) Starting from this I varied the text slightly, using "euro" instead of "EUR", also swapping "gbp" and "euro" to see how precedence affected the results. This seemed to behave as expected, but it did...
My thoughts on developing & testing.