How to clean rim brake track efficiently? What are the most efficient and effective ways to clean brake tracks on rims?
It is generally understood that rim brakes work better and pads last longer on clean brake tracks. Dirt on brake tracks tends to tenaciously sit in grooves.
Any method must meet the following criteria:

Work Safety: no exposure to toxic or carcinogenic substances
Easy Disposal: if cleaners cannot be disposed with household waste water or rubbish, or contaminate the ground in one's yard they are out
sufficiently clean: the method ought to lead to sufficiently clean rims, if your method is a wipe with an oily rag, explain why that's enough.
practical: the method needs to work with tyres on, preferably wheel in bike, and better be quick
Fold a paper towel a couple of times and stick it between pad and rim.  Squeeze the brake lever and rotate the tire.
Depending on the contaminant you may want to wet the towel, douse it lightly with a solvent, or (to get solvents off) use alcohol.