Upkeep costs on Immortal

I've recently moved up to Immortal after winning at Emperor fairly regularly. I like the challenge of it a lot, but the one thing I can't cope with is the insane difference in upkeep costs the AI and the human player pay. I have to think very carefully about placing every city because the costs go up so fast, while the AI doesn't, which usually leads to one or two AIs having huge patches of land, while I don't, and by the time I could afford the additional cities, there's no good land left to settle, and in consequence my tech and production rate lags behind. Blocking off land to backfill later is also made harder because those cities are typically some distance from your capital, increasing maintenance even more. Of course, I use all the usual strategies - go for Currency/Code of Laws early, trade around monopoly techs to squeeze out every additional tech I can, etc. Still, there are games where I've no idea how I could ever prevent the AI next to me from growing far too big for me.

I've won two games so far: One with an Organized leader and a very lucky early rush, as well as bulbing like crazy and getting to Communism very early. The other one by cultural victory (precisely because I saw I would never be able to take on the AI next to me any other way). I definitely feel that Organized is the top trait at this level. So my question is, what are your strategies for dealing with the problem, and for preventing the AI from filling up all the land ?

You have to accept that the AIs will have more land than you in the early stage, but that's not a problem. 6 decent/good cities is enough to gain a tech advantage at least on some of your opponents. Many AI don't prioritise military techs so make use of that.

You're right about the tech advantage, but I also need the production capacity. In the game I won, I took Astronomy off Liberalism and settled a yet undiscovered island with four production cities (watermills/workshops with State Property), which coupled with Globe Draft gave me just enough troops to go to war. But if there isn't any more land around, should I go all-out and destroy all those cottages for production? Or maybe wait for Democracy, but by then the military advantage could be lost...

Whip, draft, rushbuy, all ways to get up an army relatively fast with little to no hammer investment. In addition to planning your military advantage, you need to plan it in a way that you can actually go through with it and get the units somehow. Mowing Towns for SP workshops is just silly (sure, sometimes it's good on the final leg of space race, but that's another matter).

Also don't get locked into an obsession of "I must go to war at some point", going space with 6-10 cities is perfectly viable in many situations on Immortal.