1) Try to control your economy by yourself. It's not so hard, really.
Lock all goods and trade them with countries with best relations.
Choose partners with big cash and economy potential (big industry).
It will help you to buy necessary goods or resources at low prices and sell high.
Sometimes you can rob blind country selling one commodity in bulk quantities and buying this back after while but it's obvious exploit - use at your will. Best for this is agriculture or oil.

2) Try to achieve self sufficient status for crucial goods and resources: petroleum and all three goods.
Produce more then your consumption.
Rest of them could be bought from the market usually at lower prices, especially coal, ore or timber.
Building ore mines is usually unnecessary. I scrap most of them when I capture them.

3) Military bases and hydro power plants are great source of goods. Scrap EVERY unecessary site.
Hydro power plant has insane industrial goods quantities to get back when scrapping. Build more coal power plants - they are cheapest power source for almost whole game.

4) If possible build on your own land. Buildings on conquered land produce less.

5) Supply is the key. Try to have always 80-100% on production tiles. To achieve that build airfield and 3-4 supply warehouses. Or airbase but it cost more and takes more time. Use railroads to link these areas with supply center. To build in areas with 0% supply use engineers units. They also speed up building but also speed up costs and goods demand.

6) Good way to get money is to trade technologies. You can sell one tech to almost every nation so it usually worth to buy something new and sell to dozen of other countries. Best for start for small and poor countries.

7) Countries usually can spend 1/10 of their cash for goods or tech. Of course when good is critically nedded you can even get 5 times more then it's worth but this usually hurts relations.
Example: USA has 13500 M$. I can get 1350 M$ in one trade deal. Do not offer more then needed.
Day later USA has 12150M$ so I can get 1215 m$ from them.

1) Research center are needed for research - that obvious.
They are not expensive but takes very long time to build.
Use engineers units to speed up the process.

2) Usually up to 10 labs are OK. You can switch off unneeded labs at the moment.

3) More techs is being researched, the higher daily cost. Check cost to day ratio when choosing techs to research, i.e. Nuclear missile Warheads costs 6400 M$ with 171 days to research. It gives about 37 M$ per day for this one tech only. Keep in mind not to stack expensive technologies at one time. It could ruin you.

4) Choose non-tradable technologies first to research. Other can be bought from others.

5) Research efficiency is related with less days to research a tech. More spent - faster research.
It takes time to get full benefit of max efficiency. Project development generally is doing the same but it's related with costs of researched techs. If you lower - you will pay less daily but overall tech costs will remain the same. It will simply takes more days.

1) Try to control units by yourself. When you let computer to fight for you it usually ends with high loses. Units with experience are much better at combat. Don't let your units to be destroyed and try to destroy enemy units. When stacks are fighting damage are dealt only for one unit in the stack (exept of bombarding with artillery of strategic bombers) until it's destroyed. Try to withdraw and repair them.

Example:
Enemy unit is losing 50% of health and start to withdraw for repairs. Try to chase him or use air forces to destroy the unit. If not, after few battles enemy unit will get experience and will deal more damage to your units. If you can't repair your units (you are defending a city without possibility to reapir or withdraw) enemy will kill, eventually, unit by unit and capture a city while his units will continously withdraw for repair.

2) Best units are the key. Try to buy best of them from other countries and produce them as soon as you can. All depends of course of your technology. Choose wisely between production days and unit stats. Sometimes it's best to have 10 Mounted infantry units than two Rangers. Pay atention to units stats - don't produce crap if it takes long, i.e. many tanks, AA or artillery units.

Examples of good units:
1936
France - Foreign Legion and Dunkerque Capital Ship
USSR - DB3-T Tactical Bomber, T-35 Tank and Minsk Escort Ship
UK - BREN Carriers Infantry, some air units
USA - many air units but try to get P-38L Lightning Multi-Role. This unit is one of the best air units for some time.
Germany - Flak 40 128mm Air Defence (best for defence of key locations), Sd.Kfz.234/2 Puma, Sd.Kfz.234/3 Puma, many air units and tanks, Morser18 210mm towed Artillery
Others I will try to add later.

3) Don't underestimate foot units - especially Foreign legion or Rangers. In urban areas or in hard terrain they are really good. They also don't use petrol for moving so they can reach really desolate places without supply and capture crucial complex or city. Foot units also use very small quantity of Military Goods.

4) Try to fight units with max combat efficiency. If units is red (low efficiency) it will deal less damage and is more vulnerable. Let them rest before the attack.

5) Try to stack units, 7 per tile. Computer is usually dumb and attack with single units. Take advantage of that. Good tactic is to entrench in towns with good infantry (Rangers, Commando or Foreign Legion) which have good close combat attack and defence. If you support them with artillery it's almost impossible to capture such hexes.

6) More military salary ==> more reserve personnel. It's also depends on population. Don't expect to have million army personnel with 10 million population in country.

7) More Maint. & Training ===> units will get max combat efficiency faster.

8) Keep garrisons only when it's needed. It consume reserve personnel which could be used elsewhere, like staffing military complexes and unit production facilities.

9) Pay attention for units abilities. Artillery is dealing damage as well as supressing the enemy (it lower his combat efficiency). It makes his attack weaker and easier to destroy. Strategic bombers have very long range and can bombard entire hex dealing damage for all units presents. Tactical bombers (naval and land units too) can attack facilities until it's completly destroyed. It take some time and many attacks but sometimes it's good to cripple enemy's production capabilities by destroying his military or industrial complexes.

10) Use units as purposed - infantry for capturing/defending cities, tanks for breaking through and flanking in open fields, etc. If you are not sure check unit's stats, like close combat for city fighting.

11) Supply is crucial in combat. Check unit's combat time. The higher it is the longer unit have ammo for fighting. Use supply trucks or planes to supply your units in areas with no or low supply level.

Example: Marine Infantry have combat time 10 which is very low while Foreign Legion have 50.
If you plan to use Marine Infantry mind to send a transport ship (i.e. Civilian Cargo) or transport plane (i.e C130 AHercules) to restock their ammo during combat. Otherwise units will be eventually destroyed when they don't capture supply providing hex.

12) You can merge units up to 150% of original strenght. If unit's strength is around 50% merge command appear instead of split unit command. This allow to create superunits which sometimes help greatly when lacks of space. 7 units stack of Commando, Green berets, or similar units with 81 strenght in urban fight is deadly.

Example:
Infantry: 100% - 54, 150% - 81
Recon: 100% - 38, 150% - 57
Tank: 100% - 54, 150% - 81
Air: 100% - 18, 150% - 27

You have to do this manually by splitting normal unit and merging splitted one with another normal strenght unit. You can also, of course, merge damaged units up to 150 % of original strenght.

1) For trading focus on countries which are big and rich. Set support ruling party to build good relations with them. Gifts usually speed up that process. This will allow you to pay less and sell high to them (goods and technology).

2) If you plan to conquer a nation you must build Casus Belli (CB). Set support opposition as well as fund insurgency (it will lower his military support but it's not very effective). Usually you can build CB from 0% to about 70-80% in few months. Do not declare war if you have low CB. It will hurt your Treaty Integrity which impact your bond issuing capability.

3) Be very careful with alliances. You can be dragged into unwanted war in most unconvenient moment. Check also your potential target's alliances before declaring a war or you can have unpleasant surprise for being attacked from other sides.

4) You can reach far enemy with signing transit treaties with countries along the road.

5) You can raise People Opinion about your country by trading Agriculture with their country.
To raise Diplomatic Opinion support ruling party and give presents to their country.
Usually cash is the best.

6) You can support your allies by sending free or selling military units to them. Depends on game setting units will appear at once in their barracks (with 25% strenght)or it will appear at your barracks and computer should move them to needed battlezone. Sometimes it fails and units hang in your territory forever despite of being badly needed somewhere else. Devs should work a bit over that.