Please refrain from the following:
(1) accusing your opponents of cheating, whether in game or on the forum. We have a moderator email for that. You look petty and whiny when you try to slander someone who kicked your ass in public.
(2) ascribing your opponents' entire performance in game to dumb luck/your own error/otherwise discounting your opponents' skill. Doing this not only makes you look ridiculously insecure about your own ability, it just plain makes you look even worse at the game; after all, if the guy who beat you is a total bozo and you go to lengths to prove it, all you've done is go to lengths to prove you lost to a total bozo.
(3) especially if it's, oh, I dunno, 1903 or 1904 or so, don't say someone is going to win just because you got stabbed. No, they aren't, and you look ridiculously arrogant by assuming your opponent is too incompetent to see the bigger picture. If it's late game and you're holding a stalemate line, okay, sure. Or even middle game and someone is in fact about to explode in growth because you can't defend against both sides and you need to prove to the stabber that you need to live. Those are all valid. I'm talking about the Englands who end up in a war with Germany and Russia, watch France sail into NAO and ENG, and then start going "YOU'RE GOING TO LET TURKEY WIN!!!!" ...in 1903. We can see right through your shit.
(4) assuming that cheating happened because someone stabbed you. Really? You're fucking kidding me, right? Because, you know, you're God's gift to Diplomacy, and stabbing you is against the rules. This is probably the most bush league thing I've ever seen, but it happens often enough to warrant a mention.
(5) going into CD. No-brainer, but it can't be said enough.

I know ya momma and daddy taught you manners at some point in your lives. Act like it.