Its been a while so I thought I'd dust off my keyboard and write a little(a lot) post. This post may also be called "How to be a Devious Barstad to Invading Russians".
In any conflict there are three main levers that can be used to defeat an enemy.
1. Economic
2. Military/Force
3. Morale
Economic is the most important one of these. An army marches on its stomach. Deprive it of food or water and it dies. Deprive it of ammunition and other supplies and it loses its capability and will to fight. Cause the cost of an invasion to exceed the gain and you ensure that the enemy cannot win even if you also lose. Destroying infrastructure and supplies in a scorched earth policy that leaves nothing for the enemy is an ideal strategy when one cannot defeat said enemy through force of arms.
Military force simply involves making the other guy die for his country without getting killed yourself and failing that taking as many of them with you as possible if you do get killed. It also involves destroying as much of the enemy's resources and equipment as possible. War = Destruction.
Lastly, morale is influenced by the first two factors. Few supplies = low morale. Buddies getting killed = low morale. Morale is also influenced by thought and is therefore susceptible to propaganda. An enemy that believes it is doing the wrong thing by invading you isn't as likely to put their heart into it and give killing you their best shot. Constantly, reminding them they are on the wrong side, doing the wrong thing, by leaving them little messages is the idea here. Whether it is leaflets or graffiti keep getting in their heads but don't give away your position.
Some ideas for implementing these three principles in a resistance are:
Poison food or drink for the enemy to capture and then consume.
Add explosives to ammunition for the enemy to capture, yews, and ultimately have their gun blow up in their face.
Molotov cocktails are basically just anything that burns in a jar with a piece of fabric as a wick. Alcohol is your friend.
Ammonia and bleach make a wonderful deadly gas.
That space heater in the building they're using can produce a lot of carbon monoxide without being noticed.
Wouldn't it be unfortunate if the railing on the balcony of the hotel they captured happened to be partially sawed through or loose? Stairs are also an option.
Though not deadly, having your equipment fail to work is frustrating. A potato up a tailpipe or sugar in a gas tank work just as well on military vehicles as your neighbor's.
Sabotaging replacement parts is fun and requires the enemy to ship everything from home.
Some military vehicles yews tires instead of treads. Hint: sharp metal objects in the road or that thing you did with your pen knife to your boss' car in the parking lot.
Got something metal that needs burning through like oil pipelines? Try turpentine and another reasonably common ingredient I won't name here.
Technology is your friend. When possible send drones to do the sabotage/killing. Old rc cars and planes work just as well as the new fancy ones for most things.
Constantly wondering when you are going to be sniped is bad for morale. #SnipingHarassmentCampaign
Crossbows are reasonably silent.
Bolts coated in what your dog does in the yard = nasty infections.
Flour can explode. Also, any white powdery substance can fake being anthrax when you leave a note. Keeps them on their toes.
Anyone have anything else to add?