About 25 years ago, we saw the appearance of a strategy game called "Risk". The goal of the game, played by up to six players, is to conquer the world. You are a military leader, with armies, cannons and horses, and you play on a simplified map of the Earth. The game has now appeared on http://www.facebook.com
although there, they call it "Attack". Click this link to go directly to the game application.
If your game is with six players, and you use self-pick, then when it is your turn to pick a spot, pick any of the spots in the continent of Oceania (Australia). When the first attacking turn comes, you will be able to conquer and hold this continent, provided that no one who rolls before your turn, wants it. Just make sure to load up all your armies on this continent and as long as your turn comes before any immediate enemy, you will be able to wipe them out. At least three times out of four. The one time when this strategy may not work, is when another player picks one of these spots, and also has bonus armies. In that case, your attack may end up wiping out all of your own troops, while leaving your opponent some extra.
It makes almost no difference how many countries you are assigned on a continent, if you want to hold that continent. In the first few rounds, few players are willing to risk their own investment by attacking your continent. However in any game there are also one or two "spoilers" who have no continent to defend. With six players, you are most likely to be able to hold Oceania (Australia), South America or Africa. It is very doubtful that you will be able to hold North America or Europe. Asia is right out. Therefore, provided you are assigned a country in either Oceania, South America or Africa, just pay attention to who rolls before you. If they appear to be wanting any of these three continents, then you put your armies on one that they don't want. People who roll after you, don't matter in this equation. You will almost always be able to destroy them on your first turn.
Special tactic with South America
Most of the time, after you've conquered South America, you will have some left-over troops in Columbia. You want to continue on, and attack Mexico (provided it's weak) and then leave them there instead. The reason is to thwart any plans by your northern neighbor to take all of North America. You do not want a strong presence above you.
Special tactic against South America
Once players have established the continent they will control, they will want to relax a bit and simply attack in order to get cards. Four of the six continents border on Asia and typically no one tries to conquer Asia. So the holders of those four continents will simply attack back and forth in Asia to get cards. South America has a disadvantage in this regard. If you hold Australia or Europe, you want to wipe out any counties in Asia that are held by the person holding South America. What this does is force them to come through either North America or Africa to get cards. So you pit those continents against each other, while you sit back and laugh.
Special tactic with Australia
Sometimes, two of the six players both have the idea that they want Australia. You will see them build up troops there and when the first attacking move comes, one of them will wipe out the other, or nearly so, and either they will both be very weak, or the surviving one will at any rate. When you see two players both picking Oceania, you want to pick a country in Asia, close to Oceania. If you see them next both building up their troops at the assignment time. You also want to build all your troops right there, nearby. After the Oceanian Armageddon, you will simply move your powerful troops in, and clean up the entire continent, and probably make both players go botto as well.