The best place ever for good Diplomacy™ strategy that I've ever found is at:
Diplomacy-archive.com. Go check it out....it totally rocks! Of course there is always
The Diplomatic Pouch too.

Here is a small library of stregy articles on this web site for your enjoyment.

o"Cause of Tension or Cause of Alliance?" - Mr. Banning Symington
o"Help I Don't Know Any Good Diplomacy Strategy!!!" - Mr. Ryan Lux