List of factors which facilitate collusion continued..

Similar market shares: Less like one firm will feel the need to "catch up."
Economic stability: Instability, especially recessions, hurt profits and sales and will generally lead to price cuts to increase sales.
Ease with which "cheating" can be observed: If it's easy to observe your competitor's prices cheating will be less likely. In some industries where most sales are at the wholesale level it may be more difficult.