The first thing that one must know about power politics in the game or real life is that people’s emotions and trust must be manipulated for it to work, so remember that it's just a game. Don't let it ruin your friendships.
Before the game starts, say to a friend who you want to have as a secret ally something harmless, such as, "Do want to come upstairs while I grab a soda?" While upstairs, not only grab a soda but also negotiate an alliance against the others. At the beginning of the game, try to ally with the best risk player in the game. For a while don’t talk to anyone else about alliances and make your partner feel that you will never turn on them. Once they get a false sense of security, subtly arrange for another secret meeting with your secret ally.
Once you are in the quick discussion with your secret partner you have many options. The first thing that you may do is have an actual written agreement saying that you will never work with the other players against your ally. (Remember to have them sign it as well.) Secondly, you can make a pact that you won't attack the other person but "forget about" the part forbidding alliances with other people. This is very good because you leave your options open. Remember to also talk about strategy that will help you win.
A good thing to do when making a secret alliance with someone else later on and when everyone knows that both of you just had a secret meeting, enter the room where the game is being played and yell something similar to “No, I won't ally with you, and no I wont turn my back on my allies.” This is good since it enhances the sense of security felt by the person you just turned your back on. When you get back to the game say that you need to talk the partner that you ahve recently back-stabbed. When you are at a place where no one can hear, reassure your partner that you have turned down everyone's offers of alliance. Also tell him that you wont betray his trust. If he asks you to attack the person you just allied to in private, make excuses about the strategic damage it would do to you. When you are back playing the game never make a move that would show that you are betraying any of your "partners". Hopefully, your multiple allies will complete your mission for you or dominate a fair amount of the world while leaving their back open to you. (They still believe in the alliance that you are about to break.) When the time is right, strike at anyone you can as long as it breaks no promises you have made to them.
Always remember to vary your
diplomacy somewhat from game to game. Backstab everybody once, hold to your
allies the next, etc. Good luck!