Time of Day
Remember that Lawful units fight better at daytime and Chaotic units fight better at night. Ideally you want to first meet the enemy when you are strong and he is weak.
When your armies meet try to be the first to attack. Don't end your turn with units within striking range of the enemy army - let him end his turn within yours.
Choose your targets wisely
Striking first is an advantage because it allows you the choice of which units will face off. Take advantage of enemy weakness: e.g. direct your ranged attacks against foes without ranged weapons. Take advantage of weaknesses like Horsemen's vulnerability to pierce.
But remember that they get to attack back on their turn. If you are unsure of finishing off the enemy in one turn, ensure that your unit can weather the return attacks, or that you're willing to lose that unit.
Maintain a defensive line
Lining your troops up prevents the enemy from surrounding any one of them.
Note that, because Wesnoth uses hexes, a east-to-west line is weaker than a north-to south line. In an east-to-west line, each unit can be attacked from 3 directions. In a north-to-south line each unit can only be attacked from 2 directions. A diagonal line has the same advantage as a north-south line.
Use the terrain
Try to position your troops so that they are attacking from a hex with high defence against an enemy in a hex with low terrain. Note that different units work better on different terrains.
Rotate your troops
Hold a couple of units back from the front line. That way, when a unit in the front line is heavily damaged you can move him safely behind your defensive line and replace him with a reserve.
Note that your units [b]can[/b] pass through hexes containing your own troops.
Over the course of a campaign, it is critical that you build up a seasoned force. Later scenarios will assume you have level 2 and 3 units available for recall.
Your units gain most experience points (XP) from killing an enemy unit (8XP per level of the unit killed). As such, it often makes sense to have your higher level units weaken an enemy, but cede the kill to a unit more in need of the XP. Healers in particular are often weak in combat and often need to 'steal' kills in this way to advance levels.
At the beginning (when you probably have no high level units), try to give most kills to a small handful of your units. This will fast-track them to becoming Level 2 units, and they can then shepherd others.
Don't neglect to earn your leader experience. You need to keep him safe, but if you coddle him too much he will be too low level to survive future scenarios anyway.