Well, that depends on how the game develops. I often find it useful to be either head of church or chairman, if possible both. There are some agendas that give you influence and others that give a cash boost, you have a greater chance of winning the vote for them when you are chairman. It is also useful to be able to pass on the bad events and to get the good ones.
Other than that I tend to lie low and try not to make enemies, maintaining at least one large stack army near my key cities to discourage attacks. If the chance to capture a stronghold presents itself, I often grab it. Losing a stronghold is quite a setback for the player concerned. You can not earn influence from cities in the turn you lost your stronghold. That means one less contender for the throne, but you also make an enemy for the rest of the game.
In mid-game, when there are no neutral kingdom cities left, it might be a good idea to capture a neutral overseas city to avoid provoking the other players.
Also, like in other conquest games, if one player is running away with the game you have to form an alliance against him. Indeed the metagame concists of trying to convince the other players that you are not the leading player but that mr.x is.