The following options come to mind...
1.) The "Nathaniel Greene" option...
For those who don't know, Nathaniel Greene was an American general in the American Revolutionary war (1776). Greene had a small, underfed and underequipped army and was set against the British army - large, well-fed, and well-equipped. The way he beat them was (basically) to get the people in the surrounding countryside to hate the British troops more than the American troops. Every time he got involved in a straight-up fight, he lost - but he made certain to keep his soldiers from running amok in the countryside and demanding food, shelter, and sanctuary from the locals. Under the abuse of the British army and their far more taxing demands for food and shelter, the local citizenry, who had been (essentially) neutrals, grew to favor the Americans and hate the Brits.
How does this relate to D&D? Simple... provoke the enemy and get him to act in ways that alienate him from the commoners and/or the rank-and-file. Watch his power base crumble.
2.) Starvation - Similar to the "scorched earth" policy, a guerilla war against crops, rather than soldiers, can create a definite problem. Soldiers like to eat more than they like to fight.
I suppose both of the above refer to "morale breaking."
3.) Over-extension - ("You fell victim to one of the classic blunders! The most famous is 'never fight a land war in Asia'...")
This one is a little harder and requires more patience. It requires annoying or threatening the enemy long enough to cause him to over-extend his forces, then crushing those forces - or letting nature do it - before he can retreat to a position of strength.
4.) Taunting - ("Let us taunt it! It may become so cross that it will make a mistake!") This doesn't always work, but sometimes can be used to induce over-extension or morale-breaking. Usually more of a complementary tactic to some other solution.
5.) "He who lives by the sword..." Another classic example is to allow what Boromir wanted to do with the ring - simply take the enemy's uber-dingus and use it (successfully) against him (visions of Chewbacca in an AT-ST in the Ewok Battle on Endor come to mind). This works best if the weapon is somehow tied to the enemy - the destruction of one causes the destruction of the other.
6.) "Feint and thrust" - Similar to overextension (above), you draw the enemy's forces away from his stronghold, then strike a crippling blow to the stronghold or take it over entirely (#5 above) while he is away.
7.) "Elora Danen method" - Rather than having the PCs destroy something, they instead must simply keep it out of the enemy's hands long enough... if time runs out on the enemy, he is destroyed (not exactly what happened in Willow, but same general idea - the goal is to keep the baby alive at all costs). Of course, the closer the hour to the enemy's appointed death if X is not destroyed, the harder he searches for X and the more power he exerts to obtain it.
8.) The "Aslan method" - The only way to defeat the enemy is for a (good) willing person to give himself up to enemy - and be slain. The person who gives himself returns later even more powerful than the enemy who slew him, perhaps thanks to the intervention of deity. Works best dramatically for a cleric or a paladin - preferably one who does not know he will return even more powerful than before - who willingly goes to the enemy to seek the release of a loved one. (Yes, I know this one has other religious implications and figures that could be substituted for Aslan, but let's just keep it in Narnia, okay?)
9.) The "ride it out" method - There *IS* no way to defeat the evil - you must simply hide and wait for the storm to be over, then come back once evil has spent much of its power. The world of Barsaive as presented in Earthdawn is a wonderful example.
10.) The "bell around the cat's neck" method - The enemy is using a ritual to gain or maintain power. Some fool must go swap out one of the material mystic components the guy is using without him noticing; this creates an Achilles Heel for later exploitation ("the Death Star shield is down! Go for it!"), robs him of power without him realizing it (e.g., the Biblical Sampson when his hair is first cut), or destroys him instantly when he tries to use the "wrong" item, thinking it to be the right one.
I will try to come up with some more - though the "demoralize" and "Aslan" methods tend to be my favorite, the "Elora Danan" method works great as a change-of-pace from the "classic" scenario.
--The Sigil
