Averaging down (or up) on major indices over long periods of time (15-20 yrs +) generally works. Basically the shorter your time frame and the more concentrated the investment each time you average down, the riskier it is, imho.
When I think about the trades where I averaged down or held a loser way too long, it was always because I had a strong opinion. Over time I discovered the market will allow you to hold your strong opinion until you exit the trade cursing and screaming at the ridiculous price (the price you thought it could NEVER possibly reach), then it will laugh at you and move swiftly in your favor. OK, time to get back to "Trading in the Zone" so I can get rid of those market personification thoughts...