Partner with Another Developer
One good approach to testing is to partner with another developer with the understanding that you will test each other's code. Then, as you type, you will be asking yourself, "How would I test this if I were looking at this code for the first time? Would I understand it, and would I have all the information that I needed?" Some questions are inevitable, but I have found that if you know from the outset that somebody else is going to perform a unit-level test on your code without the same assumptions or shortcuts that you have made, that is excellent news! How many times have you spent ages looking through your own code to track down a bug, only to spot it as soon as you start to walk through it with another developer? This is because we often read what we think we have written rather that what we actually have written. It is only in the process of single-stepping through the code for the benefit of another person that our brains finally raise those page faults and read the information from the screen rather than using the cached copy in our heads. If you're looking at somebody else's code, you don't have a cached copy in the first place, so you'll be reading what is actually there. One further benefit of this approach is that it will prompt you to comment your code more conscientiously, which is, of course, highly desirable.