How maximize LIFESPAN of my iPhone battery

Please don't tell me how to maximize a single charge.

In order to maximize the lifespan of my battery, when should I recharge it?  When it is 80%, 50%, 20%, 10% or completely empty?

I know that maximizing NICD battery life requires you to do a deep discharge.  Is this true for Lithium-Polymer batteries?  I suspect not.  I found a suggestion that I should do a deep discharge once a month.  Is this good, bad or neither?