Make good use of your empire's lines of communication.  Talk frequently with your allies lest they begin to mistrust you -- at least until you are ready to betray them.  Talk also with your enemies, for you may learn useful information from them, or even settle your differences and become allies.  Even an empire whose power is fading can be a thorn in the side of the dominant empire if you lend a helping hand, and several small empires acting in concert can swiftly dismantle a single larger opponent who suddenly finds himself fighting on several fronts at once.  Remember also that you can send messages anonymously.  A well-placed anonymous word can make all the difference when it comes to cementing your plans.