On a similar vein, I like to partially randomize my cache invalidation period if I don't have the luxury of using some other cache invalidation scheme(http://the-robot-lives.com/index.php/2010/03/fractional-keys/). These prevents cyclic cache invalidation which can lead to interesting issues such as having your server fail every 24 hours after you initial cache restart when a huge some of 24 hour cached items invalidate and start hitting the database at the same time, or whenever the hourly, daily and short duration cache items all invalidate at the same time.
