My play has typically been to establish cordial relations with just about everyone. I think it's key not to promise something and not deliver in the first turn. This establishes as you as unreliable, not just to the other, but to the whole board. This is particularly important in face-to-face games, in which folks will see the reactions of the disappointed/surprised/upset parties.
Active, frequent communications are key, as everyone has said. In face-to-face games, this also allows you to eat up others' time, preventing opposing alliances developing.
I think it's risky to actively propose attacks with another player against a third. (An exception to this may be in some instances of the Western Triangle). Rather, establish friendly triple arrangements, agreeing not to attack each other, with broad indications of items like "so you will be heading east".