Things to keep in mind when reading this:
My doubles game of choice was 4 players, sequential, flat rate, unlimited, no fog. These tips will work on any map, but I usually played the classic map.
Feel free to post your own tips and/or elaborate on any of these points listed below. Enjoy!!!
redi5e's Code of Doubles
- In flat rate games, it is essential to pick up a card every turn. If you can’t win the easy takeout territory, a judgment call must be made: continue attacking in desperation or ending your turn and cutting your losses (getting a card is most difficult when you move first overall in a game).
Easy takeout territory advice - Find an enemy territory which is bordering 2 or 3 of your own territories. Give them each and additional army for a total of 4 on each, this will maximize your 3v2 dice rolls. - When games start to get low in terms of army count (lots of 1 army territories), make sure your teammate has a couple of 2 or 3 army territories attacking enemy single army territories (if possible). This will increase the chances your teammate gets a card next turn; 4 vs. 1 isn’t automatic.
- Fortify smart and don't waste armies; armies should be placed on the outer attacking edge most of the time (example: leaving a pile of armies on Siam when you occupy all of Oceania in addition to having 1’s on India and China).
- The key to winning doubles (besides getting a good drop, hitting mixed sets on 3 cards, and getting good dice) is efficient trading back and forth with your teammate.
- On classic map - teams usually fortify so the army count is 40-20 at the end of round 1. It is crucial to maximize attacking power every turn.
- Attack your opponents, not your TEAMMATE. There is an exception to this rule when you are going after region bonuses. It is almost impossible to hold Europe, North America, and Asia on the classic map. Africa may also prove to be troublesome so don’t waste your armies or your teammates’ armies if you can’t realistically hold it.
I live by this rule: If you can’t hold it, don’t take it, especially if you have to take out your teammate in the process. - Sometimes hiding your teammates’ armies behind yours is a valid strategy. Example: you own all of Africa except South Africa which is held by your teammate. If your teammate is weak you can protect him/her from elimination by the other team. This will also enable you to take out your own teammate to get his/her cards if this is their only territory left.
- Remember to use a singling out approach. Target one enemy player round after round until you eliminate them. The game is essentially over when it's 2 vs. 1.
- Use big stacks of armies to your attacking advantage.
- If your teammate has an overwhelming presence in a region, give him/her your armies in that region.
- In a close game, holding region bonuses increases in importance.
- Having the territory advantage is usually a good sign that you are winning.
- Always keep an eye out for opportunities to break your opponents’ territory linkage. Disrupting the other team's ability to trade armies back and forth will give you an advantage.
I don't see any secrets there 
