Well, I have noticed a flux in alliances, and that certain factions tend to keep them more often than others. Just to list those that DON'T;
~Spain
~Greece
~Britian
Now, those three factions, when playing Germania, have never once kept an alliance with me; at one point or another they will/have betrayed me, and tried to take my settlements. Currently in my campaign though, Spain is dead, and Britian is stuck on Hiberia with a full stack army that is costing too much money to save up enough to get them to the next settlement level.
But onto a current point.
I find one of the easiest ways to keep an alliance going and strong, is to have a "Boogyman" to use with the alliance. That is, to have an enemy (As someone mentioned above, can't recall the name, sorry) that both factions hate and are at war with. Usually you would want to wipe out said-faction, so that they can't bother you anymore, but that is not always the best option. Sometimes, it is better to keep this "boogyman" faction alive, so that the enemy bonding your alliance together stays alive.
For example, curently in my game my one major ally is Carthage. I've eliminated every other Roman power other than the Scipii, who after several landings on Africa to save Carthage from them (Taking cities back and gifting them to my ally), I've forced onto Sicily, and blockaded every port, cutting them off. Now, they're no longer a real threat anymore; their economy is kind of good, so they can build a ship every once and a while, but then I sink said ships, and keep what remains of their armies on Sicily, cut off. Now, they're no longer a threat, as I said, but Carthage is kept in an alliance with me, simply because the Scipii is still at war with both of us, and won't submit.
Now, one could say that the Scipii should be killed off. Granted, I could do so, since they're so weak now. But if I did that, what would keep Carthage from attacking my Italian mainland? Nothing, because there's no other threat coming at them. So instead I would keep the Scipii alive, and therefore still a "threat" to Carthage, since the AI can't tell the Scipii are nothing more than an old wolf who's lost all of his teeth. In this situation, it is beneficial to the alliance to have the enemy still alive but contained, since it still represents a threat to the both of us.
Now, there's another alliance like this. The "I-hate-you-but-we-both-hate-him" alliance is similar, but slightly different in nature. In this alliance, two factions are allied, not because they like each other, but because another faction is threatening both. Now, while a bit similar to the "boogyman" scenario, it differs in the two factions would go to war as soon as the enemy is dealt with, not needing to be defeated or destroyed. So what do I mean by this? Well, if the enemy of the two factions is no longer a credible threat on the land, one of the two allies will go to war with its former ally, since the alliance is no longer necessary. This is a bit hard to see in the game, but can happen.
For example, like above; in my current Germania campaign, The Greek Cities and Macedonia are enemies. Greece used to be my ally, but after I marched an army through northern Greece, smashed up the Macedonians and killed the Brutii, the Greeks betrayed me, since the Macedons no longer were powerful enough to challenge them. However, after building some defenses, I pulled out all my major armies from Greece, and left them to their own devices. As you might have guessed, the Greeks went off to expand in Asia Minor, and the Macedonians built back up into a power again, attacking the Greek Cities and straining their forces. Now, at this time I was off in Spain, when Greece came back to me, offering a Ceasefire and Trade Rights. I took them, and still worked on other things. A year later, Greece came back again, this time with an alliance. I took that as well, knowing they would attack when the Macedons were beaten back, but using the alliance just for some added security in the region. Now, I could march some armies into Greece, and crush some Macedonian ones, but I know as soon as I do so, Greece will betray me, attack me, etc., etc. So what do I do? This is where the "Boogyman" alliance falls into common road with this alliance. I can use Macedon for the time being to keep Greece scared enough to keep allied with me, until I can situate my armies in Spain at the moment, to get to Greece, and place them so that once Macedon is no longer a problem, I can take out Greece simultaniously, before they can attack my more southern provinces. This is over all an alliance of mutual benefits, but also of shared animosity, moreso than friendship.Now, the third Alliance I've encountered is the "???" Alliance. No real reason is given for it, and no real enemy is needed to keep the two members together. The two allies just don't bother each other, and expand in different ways. This is the most beneficial alliance, since nothing is needed to keep it going, but also potentially the most dangerous, since you never know when it'll end.
For this one, I can present to you my alliance with Dacia. We don't have any common enemy other than Macedon (But neither of us fight them, we just never got around to making peace with the northern Greeks), and we just were set in expanding in different directions, never really crossing paths (I went Southward and West into Gaul; Italy; Brittania; Iberia, Dacia expand more Eastern and Northern; Thrace; Scythia). Chances are, this alliance might end at any time (Actually, I'm planning on attacking them soon), but for the time being, we don't bother each other, don't mess with each other, and for the most part, are just partners in spreading our cultures outward. No reason for the alliance, other than it happening.
Well, hope this helps everyone here, I know this thinking was interesting to do, and even more interesting to test out in the campaigns. I'll tell you, perhaps, what Greece does does follow #2, from the signs of it, it looks to be heading that way.
Nations to play in MIITW:
Holy Roman Empire
Poland