Might I recommend ... Code Complete and Writing Solid Code? They're a bit old now, but still just as true as ever, even if a bit C-oriented. The key is to:

1. write clear code which is unambiguous
2. write proper comments where you can't make the code more clear
3. write proper documentation where you can't write proper comments
4. redesign your code where you can't write proper documenation.

If you can't write it clearly, that's usually because you can't think it clearly. And if your thoughts are unclear, it makes it much more likely that your code is incomplete (missing boundary cases, etc.).

Of course, that's all easier said than done!