I figured I'd put something together after all of the games I've played with people of all skill levels.  Essentially what this is is a guide on how to survive, be a team player, and get the most out of the game. 

Communication and Coordination...

Communication is key when playing against tough opponents, and by communicating your efforts can also be coordinated.  It is all too easy to spot a team that is failing to communicate (e.g. a 3vs3 encounter where each player focuses on a different opponent, instead of the same one.)  Calling out flags to push, towers to hit, or even enemies to focus on makes a world of difference.

As long as communication does not involve personal attacks, series of consistent messages should not be interpreted even by a quiet player as rude.  Some of the best players I've seen are the most chatty, and make sure to offer direction and guidance as to what sort of strategy should be followed.  Demigod is a team game, and much like most team scenarios a leader usually emerges.  If you'd prefer to lead, speak up early, otherwise do your best to follow the direction of an ally.

Live to see another day...

A key mistake I've seen with more casual players is an inability to catch on to when a trap is being set, or that a battle simply cannot be won before it is too late.  Be aware prior to the start of the game of who your opponents are.  If you see a regulus, expect the risk of a snipe from across the map, or mines to be stacked and hidden.  A beast will most likely invest in grab and spit, with a grab serving as both a stun and damage at the same time.  An Oak may be on top of ability interruption with consistent silence attacks, ruining your ability to teleport away or use a higher level heal potion. 

Once expectations are set, look out for any modifications of builds that your enemy may be using.  A Rook may skip towers altogether and go entirely offensive with hammer and boulder.  Modify your strategy for these situations.  Also pay attention to favor item use, such as Heaven's Wrath.  Even if you're not dealing with a Regulus with snipe, a player should under no circumstances allow their health to dip below 285 or so when playing against someone with Wrath. 

Additionally, be aware of the odds and the benefit of running versus fighting a losing battle.  Hover over your health, and your opponents for specifics on hitpoints.  Consider your ability to inflict damage, your available hp, mana, and items / potions.  Consider your opponent's abilities, such as Sedna's heal.  If you feel that you are going against the odds, there is usually no harm in retreating to the safety of a friendly tower, or the crystal to recharge.  If losing a flag is a motivator to stick it out despite rough odds, consider carrying capture locks for a simple lock and run. 

Remember, every time a player dies the opponent is fed gold (with the exception of an solo Forces kill, or a beast suicide).  It is far better in nearly every situation to spend more time retreating and pushing when more safe than it is to continue to push without retreating and die.

Consumables...

Never underestimate the usefulness of teleportion scrolls, capture locks, totem of revelations, and even universal gadgets.  These items offer a lot of bang for the buck,.  A universal gadget serves as either a 2000hp health potion, or an offensive 650 damage finisher.  Teleportation can be a life or flag saver, and can mean the difference between a win and a loss especially when portal wars occur in the late game.  Capture locks can also greatly turn the tide of battle.  Additionally, items like the Sigil of Vitality offer a 50% health boost for 20 seconds, serving as a mini-heal of sorts if used when hurt or a buff just prior to an engagement. 

Never be afraid to stock up on these items.  I personally carry 3 teleport scrolls and locks by the mid game at all times all the way until the end. 

Flag Bonuses...

Always consider the long run benefits of flags, and the effort needed to maintain control over them.  On maps like Cataract, a 15% health flag can literally dictate which team keeps map control.  Most experienced players will stack health items, and using simple math it's easy to see that an opponent with 3000 health in the early game gains an extra 450 hitpoints by simply holding the flag.  The problem is that this bonus also applies for all players on the team.  Considering that most coordinated and skilled players will stick together when it counts, or teleport in to provide support, even as little as 450 extra health per enemy can mean the difference between winning and losing.

Experience flags are also useful, however mana flags are subjective.  In my opinion, it is significantly less useful in almost all situations to struggle to control a mana flag over a health or experience flag.  Consider strategic map options as well, such as alternative methods to pull the attention of creeps if a channel is being sacrificed for more optimal flag control.  To reference Cataract again, it is quite easy to babysit the health and experience flags while eliminating both channels of creeps consistently.  If a less optimal flag is desired simply for war rank, consider using capture locks to lock and run, so that you can be available for teammate assistance elsewhere. 

Never give up...

Much like in other RTS games, people sometimes seem too willing to throw in the towel if mistakes are made.  My advice is always to stick it out and not give up.  I have seen games which were viewed as a sure-win for the opposing team end up as wins after something as simple as improving ally communication.  Try to not take mistakes too hard, as poor morale is a match killer in itself.  Instead, try to identify what went wrong in real time and modify your strategy as needed.  If an ally is being picked on, don't hesitate to offer suggestions or even provide more consistent backup.  There are many different ways to support an ally, even when not playing a true support class.

With that said, even tonight I had such a game where both of my allies were discussing conceding only to come out ahead in the end.  Some of my most satisfying wins have come from those in which I was able to make a comeback with my allies, even if they consist of AI. 

Respect...

Threats, hate speech, racial slurs, and simple disrespect only breaks down RTS communities.  Even if an enemy is venting in chat about issues such as losing an ally to a disconnect, nothing good comes from a taunt against that player.  I am seriously shocked at some of the language I've seen over the years in RTS titles, and Demigod is no different.  Treat others as you'd like to be treated, that's usually the best path to follow. 

Doubles, Triples, etc...

Rules against doubles, triples, etc. are entirely community driven.  Keep in mind that each character was designed with pros and cons in mind, and stacking characters does not mean that the game will magically become unbalanced as the negatives will remain.  Even three regulus players with mines can be easily countered with minions and a simple 50 gold item for awareness. 

Skill is what decides nearly all battles, not actual character abilities.  If there were an issue with stacking, the developers would not allow it to happen.  What I liken rules against stacking to are old community driven rules seen in other RTS games like "no rush 5 minutes" or "no superweapons".  These types of rules are usually driven by casual players, however they do not represent that the game is broken if played as is. 

If a game with a custom ruleset such as no doubles is desired, consider adding a note to the game name in order to make other players aware. 

Pre-arranged teams in custom...

Playing with friends especially in anything 3vs3 or higher is fine, but stomping unexpecting players is not.  In most cases, pre-arranged teams have an advantage with coordination, even if the opposing team consists of several random experts.  With this in mind and simply out of respect, consider noting in a game title if creating a game or in chat if joining a game that a team is arranged. 

Remember, there are several other arranged teams out there willing to play, and / or players that are confident enough to match with random allies against an arranged team. 

... I will try to add to this later, but right now I'm tired.  Feel free to add your own points.