I'm no expert yet, but I've done a lot of work recently with a Permanent Alliance (PA).
My experience is with OCC games winning by Culture or Domination. Both require a PA.
Don't waste hours playing only to find out "You're too Powerful" or "They're doing fine on their own." Read on.
Selecting an Alliance Partner
Pick someone that will not be too close and cause border tension.
Pick someone with the same religion or convert them.
Pick more than one civ in case something doesn't work out with your first choice. Be sure Choice #1 likes Choice #2
Don't trade with Civs that they don't like. Maybe a few techs early in the game.
If you're trying for Domination as early as possible, Creative Civs seem best to me.
Getting the Population is not the problem it's getting the land %, so expanding borders seems like the way to go.
For a culture victory, Creative sounds good again, so familiarize yourself with these civs
For cultural, pick a civ that's building the early wonders. Industrial trait is nice.
Gift your GP. I give Great Engineers to my chosen civ and they pop out wonders on the next turn. Of course, you better be sure they're going to join with you later. The following shows how to make sure.
Getting that Permanent Alliance
Keep your target Civ happy. Give in to their demands. You probably won't go to war unless you want to.
I play a peaceful game so I can concentrate on infrastructure and research. Maintaining a war is probably
the fastest way to get a PA, but I go for a Defensive Pact. Research Military Tradition ASAP and get that
Pact with one and only one Civ. You must make your choice at this time. A 2nd Pact can drag you into a war
which cancels your first pact. You'll need this Pact for 40 turns (quick game speed). Example, I got my Pact in 40 AD.
He finally accepted a PA in 1400 AD. I'm not sure if it's always 40 turns. It could depend on which civ.
I'm sure you have apply the multiplier for the game speed you're on.
Research either Communism or Fascism. Communism is easier and you will get there before your Pact brother
is ready to PA with you. I like this choice for Cultural victory because I like to get my PA partner to build
Broadway and the Eiffel Tower. For Domination you could go Fascism instead. You don't have to obsolete the Great Library
and you have more military techs.
Ok, so you knew most of that already. Here are some tricks I picked up:
"You're too powerful"
Look at the graph (F9) and change it to show Power. You must not be the top of that graph.
That means you can't build a big military. If you did, you can disband units or gift them to your Pact brother (warning: see next tip) or others.
"We're doing fine on our own"
Look at the Power graph again. You're Pact brother must also not be on top or you get this problem.
This means you probably can't gift him your units. If you didn't give him any and he's still on top, then you have to give
tech and probably military units to another Civ. One that you don't plan to attack probably. Just a couple units makes a
significant jump up on the Power graph, so it's easy.
Working within your Alliance
You can open their city screens (and see the culture level among other stuff).
Your workers can improve your PA civ's land.
The worker AI is pathetic -- putting fresh cottages on top of hills, etc. Change them to mines, or windmills.
Windmills have food so the AI is more likely to work that tile. Change cottages to watermills on the river.
This worked very well for Culture victory. The PA capitol is probably doing fine, but you need a 3rd city. If you
can get the production cranked up, it's more likely that city will get the Cathedrals, Broadway, Rock N Roll, etc.
Sadly, the AI worker will come along and put new cottages down again.
You can tell your PA what to research and which city to attack.
You can ask for, and get, any and all resources. However if, for example, you take the only Horses, you're partner cannot build cavalry.
You cannot culture bomb your PA cities
However, you can add any GP as a super specialist.
The Scientist can create an Acadamy.
I tried to gift a Tank to my PA once and he wouldn't accept it. Not sure what that's all about. He took a dozen Cavalry earlier.
My experience is with OCC games winning by Culture or Domination. Both require a PA.
Don't waste hours playing only to find out "You're too Powerful" or "They're doing fine on their own." Read on.
Selecting an Alliance Partner
Pick someone that will not be too close and cause border tension.
Pick someone with the same religion or convert them.
Pick more than one civ in case something doesn't work out with your first choice. Be sure Choice #1 likes Choice #2
Don't trade with Civs that they don't like. Maybe a few techs early in the game.
If you're trying for Domination as early as possible, Creative Civs seem best to me.
Getting the Population is not the problem it's getting the land %, so expanding borders seems like the way to go.
For a culture victory, Creative sounds good again, so familiarize yourself with these civs
For cultural, pick a civ that's building the early wonders. Industrial trait is nice.
Gift your GP. I give Great Engineers to my chosen civ and they pop out wonders on the next turn. Of course, you better be sure they're going to join with you later. The following shows how to make sure.
Getting that Permanent Alliance
Keep your target Civ happy. Give in to their demands. You probably won't go to war unless you want to.
I play a peaceful game so I can concentrate on infrastructure and research. Maintaining a war is probably
the fastest way to get a PA, but I go for a Defensive Pact. Research Military Tradition ASAP and get that
Pact with one and only one Civ. You must make your choice at this time. A 2nd Pact can drag you into a war
which cancels your first pact. You'll need this Pact for 40 turns (quick game speed). Example, I got my Pact in 40 AD.
He finally accepted a PA in 1400 AD. I'm not sure if it's always 40 turns. It could depend on which civ.
I'm sure you have apply the multiplier for the game speed you're on.
Research either Communism or Fascism. Communism is easier and you will get there before your Pact brother
is ready to PA with you. I like this choice for Cultural victory because I like to get my PA partner to build
Broadway and the Eiffel Tower. For Domination you could go Fascism instead. You don't have to obsolete the Great Library
and you have more military techs.
Ok, so you knew most of that already. Here are some tricks I picked up:
"You're too powerful"
Look at the graph (F9) and change it to show Power. You must not be the top of that graph.
That means you can't build a big military. If you did, you can disband units or gift them to your Pact brother (warning: see next tip) or others.
"We're doing fine on our own"
Look at the Power graph again. You're Pact brother must also not be on top or you get this problem.
This means you probably can't gift him your units. If you didn't give him any and he's still on top, then you have to give
tech and probably military units to another Civ. One that you don't plan to attack probably. Just a couple units makes a
significant jump up on the Power graph, so it's easy.
Working within your Alliance
You can open their city screens (and see the culture level among other stuff).
Your workers can improve your PA civ's land.
The worker AI is pathetic -- putting fresh cottages on top of hills, etc. Change them to mines, or windmills.
Windmills have food so the AI is more likely to work that tile. Change cottages to watermills on the river.
This worked very well for Culture victory. The PA capitol is probably doing fine, but you need a 3rd city. If you
can get the production cranked up, it's more likely that city will get the Cathedrals, Broadway, Rock N Roll, etc.
Sadly, the AI worker will come along and put new cottages down again.
You can tell your PA what to research and which city to attack.
You can ask for, and get, any and all resources. However if, for example, you take the only Horses, you're partner cannot build cavalry.
You cannot culture bomb your PA cities

The Scientist can create an Acadamy.
I tried to gift a Tank to my PA once and he wouldn't accept it. Not sure what that's all about. He took a dozen Cavalry earlier.