What is a winning strategy for Italy with Diplomacy
Make sure EVERYONE knows that Italy is the weakest country, by reminding them of it constantly, both during the game and every time the subject of Diplomacy comes up in conversation generally.  Hopefully this will lull them into a false sense of security, so that you can actually get somewhere with your beleaguered green armies.  Certainly if anyone starts picking on you, you are entitled to complain loudly: "C'mon! Everyone knows that Italy doesn't stand a chance of winning this game! Stop hassling me!"

Of course, this meta-strategy can easily backfire: if you become just too annoying and whiny, people may start attacking you just in the hope of shutting you up!  But generally I think it's perfectly reasonable, given the common knowledge that Italy is far from the strongest position in the game, to present yourself as an underdog and no real threat... until you've rallied your forces sufficiently to strike.  It's all part of the fun of the game!