There was a thread on this. Best way is as often as possible. There isnt a memory effect on Li cells. I just leave it on over night, every night usually to make sure iv got enough juice to last me all of the next day.

 

This thread should answer your question (really good read between): http://androidforums.com/htc-desire/77857-batteries-explained.html

Edit: the link should work now (sorry)

 

Sorry abou that, fixed the link I usually double check to make sure that the link I enter isn't corrupted before I post So don't know why it became corrumted