Are you playing ONE game of diplomacy, or several? Because, frankly, if you're playing ONE game, all bets are off, it's crazy, you're going to get fucked, and fucking people first is mandatory. If you play several games with the same group, you'll get to know how the other people operate, and the meta-game means that it's in your interest to never fuck an ally in the first few games, or always tell an ally when you break alliances.
Why? Because you will develop a reputation as the Trustworthy One, and while everyone else is backstabbing the fuck out of each other, you highly increase the chance of profitable alliances. Until they last game, where you absolutely RAPE your poor, stupid, trusting allies.