Uncle Genghis's tips:
1) When you're just starting communicating well is important - by talking through strategy you will spell out the thought processes behind your decisions. As you gain experience you will no longer need to discuss strategy much with your partner.
2) Work with your partner. Your partner's troops are yours and your troops are your partner's. Exchange troops freely, don't be afraid to make sacrifices for the good of the team, and expect the same from your teammate.
3) Know your partner. Everyone has a different style - play lots of games together and you will get to know that style, and you can predict what your partner will be thinking. You can then make decisions based on that which will result in the best use of your troops.
4) Be aggressive. The dice are supposed to favour the attacker... so attack! In team games (excluding 6p doubles) it is your team against theirs, so you have nothing to gain by holding back potential attacking troops. However, there are times when it is wise to hold a little back, you need to learn when those times are from experience.
5) Think ahead. Consider everything that may happen before your next turn and your partner's next turn and try to counter it/use it to your advantage.
6) Play lots of different maps and settings. This is the best way to improve your game, you will learn the need to be flexible and how to devise new strategies on the spot. This will then help you to be creative on the maps and settings you are better on, which could serve you well in tight situations.
That's probably a good starting point

Everything else you need to pick up 'on the job' so to speak.