If 9/10 times adding to a loser works out fine, why would you "NEVER ADD TO LOSERS"? It would seem this is exactly what you should do (90% is a great winning %). The real key is to understand when that 1 time is upon you and finding a way to take your loss without it being a disaster.
Quote from makloda:
Make sure you always remember this lesson. 9 out of 10 times adding to a loser will work out fine and you get out with a feeling you got away with murder. But the one time comes - sooner or later - that makes you regret you ever started playing the game.
Increasing your position is fine, just make sure the position is in the green in the first place. NEVER ADD TO LOSERS.