Watching the Enemy
It is important to make periodic checks on the enemy town to see what units are available to them, and possibly what strategy they might be trying for. It's important to become familiar with the tech tree for all four species, even if you specialize in playing only one of them so that you will know what they are doing and how to counter it. Stopping various strategies is often as simple as catching the enemy while they are attempting them.

Starving the Enemy, Deny Expansions
Often winning the game is as simple finding an enemy expansion. It is important to grab multiple resource spots in order to build more units, or to afford the more expensive units and upgrades. If you can deny the enemy these additional spots, they will not be able to fund further troops and you will eventually be able to overpower them if you use your troops effectively. Many inexperienced players don't realize this, and when they lose the game, they wonder why they lost. Often the reason is that their enemy mined more resources than they did. Take a look at the stats and the replay to see why you have lost.

Goblin Zeppelins
Goblin Zeppelins can be purchased at the Goblin Laboratory early in the game. These are perfect scouts and can be used to carry your troops around. Human Scouting
Use Flying Machines or Gryphon Riders to scout the enemy. Use Mechanical Critters available at the Arcane Vault. Use Mortar Team Flare or Arcane Tower's reveal. Orc Scouting
Orcs can use the Blademaster's Mirror Images to scout the map. The Far Seer's Far Sight is a good way to scout the enemy because there is nothing they can do to prevent it beyond killing the Far Seer. The Witch Doctor can also use Sentry Ward to watch an area after you have left. Place Sentry Wards at expansions.

The Far Seer's Feral Spirits are also good for scouting. They can also be used to harass workers.

Night Elf Scouting
The Night Elves have several flying units that can be used for scouting. Huntress Sentinels work very well for scouting. Place them around the map. The enemy rarely tries to remove them. The Priestess of the Moon's Owl is also very good for scouting since it cannot be destroyed.

Undead Scouting
The Undead have several flying units that can be used for scouting.

The best scout for the Undead is the Shade because he is invisible and can see invisible units.

Finding an enemy that is hiding

Toggle Minimap allied colors
First click on the crossed battle-axes button to the right of the minimap. This will turn the enemy red on the map. This will allow you to easily see enemy buildings and units on the map when they travel through areas you can see.

Reveal the Map
Use everything at your disposal to reveal every bit of the map. Once the map has been revealed, the enemy won't have anywhere to hide. This is not hard to do using flying units, ground units, and spells.

Orcs and Humans can build towers around the map. If the enemy tries to travel near them with their worker, they will be killed.

Use a Goblin Zeppelin to scout or use the Goblin Laboratory's Reveal option to reveal the map.

Orcs: Use Wind Riders, Witch Doctors to put Sentry Wards all over, and Far Seer's Far Sight.
Humans: Use Flying Machines, Gryphon Riders, Arcane Tower's Reveal, or Mortar Team's flare to scout the enemy.
Night Elves: Use Priestess of Moon's Scout, Huntress Sentinels, or Hippogryph Riders to scout.
Undead: Use Gargoyles or Shades to scout the map.
Build flying units
Build flying units so that you can easily and quickly scout the map. Use waypoints to set up patterns around the map.

Use Invisibility Detection
If the enemy keeps doing hit-and-run attacks with their Hero use invisibility detection to find them.

Defend the Neutral Buildings
Build Towers at the Neutral Buildings and cover them with invisibility detection. This will keep the enemy from using them.

Catch that Goblin Zeppelin
If an enemy is using a Goblin Zeppelin to scout, chase it down using flying units. It is also helpful to use spells such as Ensnare and Web to cripple the Zeppelin.