I discovered a video presentation available from QCon SF 2008 by Neal Ford called 10 Ways to Improve Your Code. Although the examples are in Java, most of the presentation is relevant to PHP. There are some ideas here that I've never heard of myself, such as "anti-objects". Some of the main headings are a bit cryptic, so you may have to see the video itself to have them explained.

The list of ten ways includes things like test-driven development, good citizenship, the "single level of abstraction principle and anti-objects.