So i've got to say I find that I'm disagreeing with you guys who said it's impossible to keep an alliance afloat.
Ghandi and I have been best buds since the beginning of the game. It's 1900 now and we still have a very strong relationship, dispite his just passing me in landmass and cities. Time will tell if this holds true, but I expected him to turn on me long ago but he's still very friendly.
One thing I have to stress is to NOT horde Gold if you're working with an ally. I had stockpiled 20,000 gold waiting for a tech so I could do a massive military upgrade. Then Ghandi comes over
"Hey bud, could you borrow me 7,000 bucks? Mkaythanxbai." Since I wanted to keep him as an ally and not get a negative for not helping him, I had to give it to him. That sucked. Lesson learned.
But moral of the story seems to be that as long as you're willing to bend over backwards, give a little gold here, and a luxury resource there (better to sell them as soon as you get them, so they can't ask for them for free) it's entirely possible to keep alliances in place, even into the late game.
I suppose one could say that it would be easier to just annihilate him, and that's true, but I'm going for a diplo or cultural victory here. So having 1 less nation to worry about really helps. We have a kind of canada/usa relationship going on here, with the longest undefended border in the world. Really frees up my military to focus where it's needed.
I just really hope I don't end up being wrong, and get screwed.
I'll let you know how it turns out.