Always aim to put yourself in a position that you can easily conquer at least one country every turn. This way you do not have to spend a large number of armies just to collect a single card. In the initial stages of the game, this is usually not an issue. However, towards the end, one-army countries become scarce. A common technique is to leave a one-army exchange country close to your borders and share it with other players. Each turn you can attack it with minimum effort and get a card. Other players will follow suit as everyone is happy collecting a card every turn. This tactic is also known as card farming.

The exchange country also works as a pressure valve. If someone is unable to easily get a card, he is forced to start a massive attack on one of the fortified borders so he can get his card. Anyone can exploit this to his advantage. You or your enemy might conquer the exchange country and leave a large army behind. This will almost certainly annoy other participants of the exchange country scheme. The happy days are effectively over! Make sure you know what you are doing. This scheme is illustrated below. Afghanistan is exchanged turn by turn and acts as a pressure valve since all borders between players are very strong. Without Afghanistan they are forced to spend a large number of armies to get a single card. 

The owner of a territory that has access to numerous continents can exercise a great deal of power over others. Likewise, he will also be subject to attacks from others as they may need to get through him to get to their goal. Hence, attempt to be the first person to conquer such territories to maximise your political gain and lead the strategic attacks.

There are a number of choke points, or so called intersection grounds in the game that are absolutely critical to control. Owning them gives you more options when you want to attempt surprise attacks or respond quickly to enemy army rearrangements. A classic example is Middle East. This territory has access to three continents and a large army in this strategic location can threaten players in Asia, Europe, Africa and even Australia. There are of course other intersection grounds.

Please vote on where you think these intersection grounds are.