I will constantly update this thread with links to new strategys (possibly demonstrated in video or pictures) and tactics you can use to get ahead in Campaign, Skirmishes, or Multiplayer. (for MP, how to counter as well)
I'll be making a website later dedicated to the listing of strategies, and demonstration of them.
First off, lets discuss some to make a list.
I'll just throw some general ones down right now.
Always build a 5x5 lair, considering you usually might build around the lair so it would be hard to expand it without building a seperate one.
A 4x4 hatchery is prefered.
If you are on a map with plenty of gold, or even gems, be sure to build a small combat pit for fast training. Using a combat pit compared to a training room can double or even triple the speed of training your creatures, not to mention, it doesn't use up gold like the Training room.
Be sure to expand early, mining paths to special units, items, or rooms so you are able get an edge. If there is a small amount of gold on the map, get it before your enemy does, and he won't be able to expand as fast.
Always build the prison away from the front of your dungeon, to keep the enemy from jail breaking your prisoners and gaining reinforcments.
If you are able to, dig around the main path and flank an enemy base.
Use skeletons as a last resort or a final attack because they do not pass out when they are near dead, they will just die.
If the enemy possesses a unit, and pushes to the back with it to attack your support units, either lighting it or pick your men up.
Be sure to build atleast a braced door between each room, if not steel. If you have plenty of money and several trolls, build magic doors.
I'll think of more tips and strategies later. Feel free to add to this.