I think the best training for approaching a Diplomacy game might be martial arts competition. Basically, competitors should respect each other, and that means agreeing to fight like hell and do anything within the rules to win while in competition, then not to hold grudges when the game is done. If I'm playing against you, I want your best shot, you want my best shot, and we see who ends up winning. The ground rules for Diplomacy say that lying, playing to logic, playing to emotions, screaming, making threats, and all that are part of your toolkit, and are to be used (as long as they're within the context of the game). If you repeatedly lie to me, threaten me, etc. in-game, I won't take that personally when the game ends (although I will take it personally in-game if, and only if, I think it's to my advantage to do so!)
If it helps, I checked a sample of my press games once (I play mostly gunboat on webDip, but not solely gunboat) and found that I only lie about 2-3% of the time. I don't know what the optimal ratio is, but I've found that, much like prisoner's dilemma simulations, the more duplicitous tools in my arsenal are best used sparingly. But, if you're don't have fun using those tools at all, then this probably isn't the best wargame for you, especially when there are other well-constructed games that involve different tools.