Whether you’re looking for a tribe or trying to lead one yourself, you need to be able to identify what a "good" tribe is, and what qualities you should be looking for in it.
In a word: Activity
Nothing can happen without activity. If the members and especially the aristo aren't active you’re sunk. I don’t care if you have 100 of the best TW players to ever grace the game, you will have a failure of a tribe if they don’t log in. This may seem like a no brainer, but you would be surprised how many tribes have numerous yellow or even red players. I'm not just talking early world either, late in the game when players start becoming bored it becomes even more prominate. A million point account doesn't do your tribe any good if there isn't anybody at the helm to control it.
As a leader you need to recognize this and kick the offending members. Find a new active player who will fill the void. It may not be the best for the tribal rankings, but when it comes wartime you will be much stronger and much more capable. Further having active players will keep your internal forums more active and better maintain the attention of the players within a tribe. Dead forums will often translate into a dead tribe. Further this activity is necessary for further tribal needs like effective communication and ability to war.
Next thing that is key for a tribe is to make sure it's tactics and strategy matches your own play style. An active aggressive player isn't often going to be happy playing in a K tribe that recruits or merges its rivals rather than wars with them. Likewise a defensive player isn't often going to be very comfortable in a spread out aggressive "elite" tribe. This also includes such things as proximity of tribe members, coordination between players, and forum(in game and world) presence. As a player you need to find the sort of tribe you want to play in.
Also make sure your tribe's goal matches your own. If your in this game to have fun play in a tribe that will allow for that. If you’re in it to dominate and outlast the server, join a tribe with similar goals. There are many ways to "play" tribal wars successfully make sure your tribe works with the way you want to play. I'm not here to argue that one type of tribe is better than another. Just be sure that the type of tribe you join matches your playing style. If you don’t enjoy your tribe it’s going to be very difficult to enjoy the rest of the game. As a leader you need to recognize it and recruit the type of players that match your tribe's "style".
Next big thing for any tribe to do well is communication and along with that the community of the tribe. If tribe members don’t enjoy one another company they aren't going to stick around or be very willing to support and coordinate with one another. You want the desire to assist one another within the tribe to be a natural response because of a friendship within the game, not because the duke or war commander or whatever ordered it. Whether that communication occurs over igm, forums, IRC, msn, skype, or any other number of places doesn't matter. What does matter is that the tribes members get allow and want to see one another succeed. As an aristo there isn't a lot you can do about this. Either its there or its not. The only real influence you have is in recruiting the "right" players for your tribe and in arbitrating internal issues before they tear the tribe apart.
Continuing along, the aristocracy of a tribe can make or break a tribe just as easily as the member base can. Even quicker at times depending on the situation. There are several things to look for when determining the strength of the aristocracy.
First the aristocracy should be developed out of the leadership of the tribe, not the other way around. That is to say that just because you give a player "baron" status it doesn't make them a leader. It’s the respect the members have for said player and their willingness to follow that makes them a leader. Leadership leads to the aristocracy, the aristocracy doesn't make you a leader. That being said it also is common that there are several leaders within a tribe that don’t hold official rank yet are very involved in decision making and policy within the tribe. Often times these players are some of the best leaders in the tribe and can hold more "power" over the membership than even the duke.
Further the aristocracy should realize that they are there for the member’s benefit and that without said members they wouldn't have a tribe. Many a good player becomes a bad duke when they take the reins of a tribe and try to act supreme overlord. It seems somewhere they forget this is a game and those people playing the account are doing so for entertainment, not to further inflate some oversized ego. The leadership should listen to members concerns and not make selfish decisions in disregard for the membership of the tribe. All it leads to is alienation of your player base and in the end you will likely cause a split within the tribe. That isn't to say that you need to have a "democracy" within the tribe. In fact I have found that most democratic tribes have the worst leadership of all. They are slow to react and, without central leadership, often incapable of properly coordinating themselves.
The leadership needs to be strong, respected, and above all active. If the aristo isn't around can’t lead. And a tribe that isn't lead will inevitably fail. The leadership is there for a reason, being primarily, to coordinate and unify the tribe for developed goals. If any part of that is missing the leadership is failing for the tribe, and the tribe will in turn fail for the leadership.
In diplomacy a tribe shouldn't have more allies than it needs. While a long list of friends who will "fight" for you looks powerful it has very negative consequences that many don’t weight properly before they develop an alliance list. Further many tribes don’t consider the lasting ramifications of said alliances when they are first signed. What looked good 2 months ago can often change overtime.
First each and every of those "allies" is a war waiting to happen. Those allies are inevitably going to cause conflicts as they expand into outside tribes and eventually they are going to call on you to join them. You’re obligated through the agreement to back them up. Oftentimes it will be at one of the worst times, while you yourself are fighting your own wars of expansion. But in all but the rarest of circumstances of this scenario you’re looking at a situation where you just earned yourself a new enemy. If you refuse to enter the war then often you’re looking at even worse consequences in P&P and backlash from your former "allies".
Second every one of those "allies" is one more group of players you can’t expand into. Not that its always bad, but if you have prime targets in an allied tribe it isn't often your going to be allowed the opportunity to take them. This will in turn force you to attack less opportune targets. As a result proximity and relative power should be considered when making alliances. You need to pick tribes that will offer you more benefits as an ally than as a target. If you could easily take them over they are likely not a good ally and you better off putting those villages under more capable tribal affiliation, namely yours.
Next you need to make sure your allies get along, with each other and with you. A lot of damage is cause with relations when you are forced to pick between 2 allies because they went to war with each other. Few can play the "neutral" card and still be respected in the end. Further even if you and your allies don’t all get along its going to make coordinated efforts much more difficult and further inhibit the war effort. If you can’t count on support between your tribes because of political issues then they certainly aren't the right sort of allies.
Enemies are just as important as allies. You always want to have somebody your tribe is attacking. Idle time for a tribe is death. Players get bored and will leave, and in general you will see your tribe fall quickly into decline. You need to grow and attack to remain a top tribe. If your leadership is always waiting for others to try to take them on they will fail. Further a tribe that always grows through mergers and assimilating players they will rarely maintain a good member base that knows how to handle wartime situations. Because when you get down to the bottom of TW its war. In picking enemies you need to weigh several factors.
You want an enemy that’s near enough your tribe can attack easily. You don’t want to have to launch attacks halfway around the world. Further if they are near you they will offer more strategic territory that will assist the tribe in the future. A tribe with a single large core will have an advantage over tribes with many clusters. Further your tribe is going to lose interest if they are all blued in. The fighting is what makes the actual game fun.
You also need to weight the allies of the target and their forum presence. A tribe with more allies and a better forum presence will bring in more outside assistance. Likewise you need to evaluate your own allies and forum presence to make sure that you can handle your enemy. It’s not unknown in TW for a tribe to wipe out another in a war but to then be destroyed in the following months because of the targets allies and forum support.
Also you need to weight the skill of the players of the target tribe vs. your own. Understand that although a tribe may not have the points of a larger tribe their smaller tribe can often consist of much more skilled players. Never underestimate a tribe because of their member count or point total. Evaluate you target carefully. Nothing will destroy a tribe morale and forum presence like a large cocky ranked tribe loosing a war to a small elite tribe that knows how to handle itself.