Battery lifetime

How long do Li-ion batteries typically last before they stop holding a charge?  I pulled one off the charger today and was able to take about half a dozen pictures before it showed totally dead on the camera. 

On the same note, any tips for making them last longer? 

This on is probably 3 or 4 years old, so it's not surprising that it is going bad.  I'd just prefer not to have to buy new ones if there's a way to make them last longer. 

My camera batteries are all a couple of years or more old and all three will take a thousand photos less if continuous focus is on and pop up flash is down.

Penlights for speedlights I give away and replace them all every spring. (two sets each for four flashes)