Before posting a modeling technique, I look to see if it can meet two marks.  First, it must be simple enough that an entry level modeler (middle school student, recent retiree, etc.) can easily employ it.  Second, it must create a fair degree of visual impact.  Panel line accents meet both criteria.  Forcing shadows into the crevices of your models makes a tremendous improvement in their overall appearance and is a very simple technique.  Essentially you are wiping black over the crevice and then wiping off the excess.  Tamiya even offers a dedicated product for this.

If I had to pick one area where I’d apply this technique to first it would be the side panels on a diesel loco.  As with anything, do a few trials runs on a scrap first and then move over to your actual model.  This isn’t a marginal technique to employ on occasion but really a standard that should be employed on most of your models.