Mad props to Kevlargolem for his competitive guide (viewtopic.php?f=5&t=1470), but what happens when reputation is REALLY at risk, for example against a forum user (especially those games which are started and hosted through the forums like TET, tw2000 tournament, ESG, etc)?
Here's my take:

Day 0 Diplomacy - Good ol'days
Chances are you probably have seen most players before, so chat up past games and experiences to get your relations rolling. Even if you pulled the rug from under them, or vice versa, keep conversations pleasant and humerus. As with most games here, go for at least 5 outposts in the beginning in a 10 player game - there are 99 outposts, so having 5 puts you in the average. Calculate accordingly in games with less than 10 players, and strike deals for more if possible. Since the other players know each other as well, consider using past occurrences to convince a player to ally against someone, knowing they are a ruthless/deceitful person. Reach out to friends that may not share borders. Some players are pretty chill, so at least they won't go to war with you the first few days (personal notes are important here).

General Diplomacy
War will seem more civil than normal games (unless a mortal nemesis status is achieved), and as is with normal diplomacy, lots of talking will be present. Unlike normal diplomacy, you will have to put more effort into making a good reason for war (you don't want to be known as the guy who gets mad at everything), although an apology after the war/game is always appropriate and can dampen any anger in your relations. As many players will put post game accounts on the forums, it is a good idea to comment on them about the reasoning behind your actions, as not to appear completely reckless.

Mortal Nemesis, Chillbro, or Friend?
I have mentioned the mortal nemesis before - this is the person that either hates you, or you hate because of whatever actions have been caused in the past or in the forums. These may include general nastiness or repeated cold-hearted back-stabs or deals. If your mortal nemesis is in the same game, it is in your favor to convince other people of his reputation and to go to war or at least be wary of him. If sharing a border, it is in best interest to wait for him to strike first to legitimize your revenge. The Chillbro is the kind of guy who you know won't go for an early assault against you, and keep peaceful with you for most of the early and mid game. Chances are he is building up and mining. Come mid-late game, he may write you a heartfelt apology for a declaration of war, telling you how sorry he is but how he still wants to get first place. (Remember, placing top3 is still a priority over good relations, as explanations and apologies can come later) It is then when he unleashes his built up forces against you. To counter this, have at least one ally bordering chillbro, either to keep him in check or to help fight. When considering to ally with chillbro, don't expect too much help - he'll donate some drillers and take maybe one or two outposts in a ware you're involved in. The Friend is, well, a friend. Your relations have reached high enough hat you'll either stick allies for early-to-mid game, and when declaring war against each other is always good-natured. Strategic maneuvers are lauded, and the war will usually be to elimination, because who wants to lose face to their friend?

Tournaments and Series
What happens when you're participating in a forum tournament? Each player will be known, and you may face a few again in the same tournament. In this case, notes are important again, as you want to record their known moves, as they are likely to be seen again. As always, talk with other players to gain intel on other players' habits and play-styles, especially on Day 0. Furthermore, the higher you rank in these tournaments, the more powerful and threatening you will seem, giving you the advantage of being able to choose allies, but also being allied against.

Too long; Didn't read
Always be courteous, keep notes on all players, use past experiences to make allies against another player.

Please comment any other tips and things that could be added to this guide!

Kind of like relationship strategies. Kevlar's post matches with yours.