When starting a campaign, build at least two or three Farms or Slaughter Houses. Also, make sure that all your men are guarding the castle (put them all around the base), so that no enemies or trolls attack you.
Hordes/Battalions with a Banner unit should be retreated when only a few remain to allow the ranked unit to regenerate.
Always close your Castle Gate and build a Postern Gate instead.
Try to grab as many of the economy/outpost plots surrounding your base.
Fewer upgraded units are always better than many non-upgraded units.
Build two economy structures per one unit production building.
Build economy structures in the back of the Camp/Castle and build unit production structures in the front.
Small cavalry is better than a large infantry, except against archers.
Do not blindly put any group of soldiers against any enemy soldiers. If you have cavalry, make sure to have archers as back-up against pikemen and the like.
In skirmishes, build a wide variety of hordes, but mainly use Archers and Monsters above Foot Soldiers. Always have all Heroes before an attack. Keep gates closed and heavily guarded. When all allies are defeated, use specials to summon units like Summon Ent Allies or Rohirum Allies. When raided, build as many Orc, Oruk, Gondor, and Rohan archers as possible. When you are outnumbered and other units that you control are far away, run away.