Leaning until rough running, then richening the mixture works pretty well with small singles like the PA28.
If you have an EGT guage installed, then the guage has a moveable redline marker - after accomplishing the above general procedure, you move the redline marker until it is over the white EGT indicator, then lean a smidge more, and wait 30 seconds or so. If the egt has increased past the redline, adjust the redline and do the same again. Continue this until you find the peak for that operating condition, then set either rich or lean of that line as required.
The temperature you are chasing changes with different operating conditions, hence the moveable marker.