Nearly every game I win is a diplomacy victory (monarch level), so let me tell you how I do it.
I am extremely careful with my reputation. This is the first and foremost thing, because if you trash your reputation no amount of gifts and bribery can fix it. Taking care with your reputation tricky. Some of things I do include
*never, ever razing an AI city. Ever. When at war I use artillery to bomb the population down and after I take them I starve them down to 1 pop point; with strong culture and a strong garrison I never lose captured cities to culture flip this way. The AI doesn't mind me bombing and starving them to death, but razing an AI city will destroy your reputation.
* never, ever engaging in RoP-rape, or the other actions that the AI treats as RoP-rape. That means making sure you have no units in their territory when you declare war - not even a lone ship a few squares away from one of their cities. I declare war on the diplomacy screen either by choosing "prepare for war" or by calling up the peace treaty from "active" agreements and canceling it.
* never turning down a deal without offering something else in return. When the AI offers me a deal I do not want to take (like "World Map" for "World Map and Combustion"

) I say no, but then give them the world map for free (for example). This is the only time I use gifts.
*never, ever, break an existing deal. If I want to wage war, I wait until active trades with the target civ expire. Then I call up that trade and cancel it. Then, and only then, will I declare war, again making sure there's no RoP-rape.
*a corrollary to the above is, never entering MPPs. I want control over when and with whom I go to war, because breaking deals trashes my reputation. So I simply do not enter MPPs. I will grant RoPs to civs that ask for them, but I take MPP off the table. I will also use military alliances when I want to draw another AI into the conflict to force my enemy to fight on two fronts. When I do that, though, I always wait 20 turns and end the military alliance before making peace, in order to avoid breaking a deal.
*Finally, understanding that not every civilization will like me. I usually wage two or three large wars a game where I cripple my nearest neighbor or strongest rival, to get their resources or just to make sure I am the leading civ. The civs that I fight with will never like me, no matter how honorably I conduct the war. That's okay - I don't need every vote, just a majority.
As you see, it takes some work and care to maintain your reputation. But when I follow the above steps, I will nearly always win a UN election. (I have never lost one, although they are sometimes inconclusive.) I also follow a generally peaceful builder type of strategy, with a few strategic wars sprinkled in, and thus by the time Fission rolls around I have a healthy tech lead and never have trouble building the UN.
I'm interested in feedback on my comment. I didn't realize when I set out that I'd have so much to say - but since this seems to be my main mode of victory, perhaps I will repost this to the strategy articles forum if people find it useful.