Starting in 2000, narrow green painter's tape/Sharpie placed on the bench top in front of the carboy. Moved on to paint pens, using last year's queen color on the carboy proper. Moved back to painter's tape, the wide blue stuff and Sharpie or paint pen, still on the carboy. Sometimes it's a mix of both.
The longer we're at this, the more information we want on bulk aging. Two inch painter's tape no longer provides enough space to get all the info (see Billdean's post above) that we want. That necessitates using several strips of tape, some that can move between containers when racking.
As we move to larger containers (and less variety) it's getting easier. Stainless containers and barrels offer a lot of space for data.