I noticed that if you have less than five Risk cards at the beginning of your turn, and they will make a complete set, you can gain many more armies than the next level of award.
Go to the Risk card screen and select which ones you wish to trade in. The game will automaticaly take you to another screen showing the set you turned in.
When the game then goes to "Pause Menu" screen, press the 'X' button again to return to Risk card screen. Select the same cards again, then go back to Risk card screen.
You can continue doing this many times. What happens is that the games awards you reinforcement armies just as if you had turned that amount of sets of cards. For example, if you trade in a set for 4 armies, then use this cheat, you get 4 armies, then 6 more (for 10), and 8 more, then 10, 12, 15, 20, etc. I have recieved as many as 633 armies on turn before.