Use decoys. The computer will often fire at trucks, transport trains and transport ships (even empty ones), construction units (even spent ones) and sappers. None of them are typically your most valuable units. Keep such units, for example empty trucks, within the range of enemy artillery to draw fire, especially during an offensive.
Take prisoners. If a single enemy units approaches an otherwise isolated depot or factory and you have units on the outside, you may let the enemy capture the facility and recapture it during the following turn, gaining the enemy unit inside it. The recapture must take place within the following turn, though, since otherwise the enemy will have time to move out with all the units that he himself has just captured. Make sure your units actually have the capacity to capture buildings: infantry does have it, but armor does not. Additional, deploy or (re-)capture a building with units, that does not have full health points, straight before the computer occupies this facility. It usually will repair all that damaged units on his costs.
Double your rate of fire. If you fire at distant targets, the enemy unit will not shoot back during your attack turn; if an enemy unit fires at one of yours and the two units are on adjacent hexes, your unit will shoot back in defense even during your movement turn. Since the computer tends to fire at point blank, thereby drawing return fire, you can achieve double his rate of fire by firing only at non-adjacent targets. The effect is especially prominent in naval battles, where your gunfire will never cease while the enemy will only fire every other turn.
Shoot down aircraft. Enemy aircraft are dangerous as they can move over long distances and fire at your units with impunity. When one of them is near your lines, try to surround it, even using units that cannot actually shoot at them, just to prevent them from escaping. Within an attack turn, first shoot with AA emplacements, ideally from a distance so that the enemy aircraft do not shoot back, weaken you and needlessly gain experience. The range of armored trains and stationary flak is 2, the range of mobile flak is 5 hexes. Then shoot with elite infantry, which can engage airborne units very effectively but are still cheap enough to produce and repair. Only then should you let your own aircraft engage the (already weakened) enemy, since they are among your most precious units.
Let them gain experience. If two artillery units could potentially fire at an enemy unit, and one of them has maximum experience, let the other one shoot first. Otherwise, the veteran battery may well wipe out the entire enemy unit and the novice battery will have no chance to increase its experience. Also wisely engage enemy units which are down to only one member. The annihilation of an entire unit through the destruction of its last member after all adds two experience points instead of just one. Keep such rewards for new units, do not waste them on units that have enough experience already.
Use target practice. If you let inexperienced fighter aircraft attack enemy armor, artillery or bunkers right away, the result may well be that they will not destroy a single enemy unit. This means they will not gain any experience. Let your aircraft engage ‘soft’ targets instead, such as cavalry and infantry – not elite infantry, though, since they will actually shoot back. Direct them against harder targets after they have gained experience.
Use supply chains. To move units over long distances, you may sometimes have an opportunity to use several transport units in a chain. For example, a train may move and drop the unit it is carrying onto a waiting truck, which can take the unit further by road, all within the same turn. Estimate the range of transport units to put other transport units into the right waiting positions.
Building 'Bridges'. Some unmoved Transport-Units in a row (without gaps) could be passed by other units like a road, even if it isn't possible to load the passings unit into a truck or transportship. This way, it is possible to move an armored train over a road (full of trucks) to another piece of railway. Or move some infantry over a strait of water with the help of transport ships. It is important that the moved unit has a place to stay at the end of the bridge, like a railway, road or an empty truck (for e.g. infantry, artillery). Strangely enough, ships can't pass any kind of bridges but if you place a transport truck on top, they can.
Stay out of the artillery’s range. Remember the range of artillery units and count the hexes on the map to see how far you can advance without drawing fire, or when exactly the enemy will start taking your lines under fire. Anti-tank guns and armored trains have a range of 2 hexes, bunkers and light artillery 3, medium artillery 5, heavy artillery 6, rail artillery 7.
Use Construction Units ability to block passages. Build a half depot (remember: it takes two rounds to complete a depot) at locations of strategic importance, but do not finish the building. That way you tie up free traffic of computer units, without giving him a chance to capture another depot. However, sometimes it may be better to complete the building. Note that you also impede your own free movement, and that this procedure is irreversible, so you may no longer be able to reach areas of interest or even to beat the game.
Encircle enemy units. Use two or more units against one enemy unit. A unit that is attacked from two opposite sides, will most likely be destroyed.