It's also very beneficial to educate oneself in formal logic, and deductive reasoning. While Diplomacy is a big part, I can't help but think that a lot of player's tactical questions are easily resolved just by understanding syllogisms.
Otherwise, the Diplomacy Archives are excellent, although be sure to understand that phpDip rules are a little different from the ones that they are discussing, which are the official rules. Also keep in mind that they are often referring to postal, play by e-mail, or even face-to-face games, which may be very different to the play seen here at phpDip.
The greatest vice in Diplomacy is predictability. If you let everyone know your intentions, or you make consistently predictable moves, alliances, and stabs; you will lose.