Dan Harrington has written a book about poker in which he introduced a system for online poker tournaments. His book is called "Harrington on Hold'em, Volume II: The Endgame". The system this book brings forward is called the Zone System and it basically splits up a tournament into various zones that depend on the stack of each player in comparison to the antes and the small and big blinds. You will enter a zone when you get the appropriate value to fit in it and you will play in each zone accordingly, applying different strategies for each and every one of them.
Harrington divided the players by a criteria based on comparing the amount of the money players have to the starting pot. So if you have $ 1500 and the blinds are $ 50 / $ 100 and there are no antes required in this game then your money is equivalent to 10 times the starting pot. This value has been referred to by Harrington as the zone indicator and it has been abbreviated as "M".
Zone 1 - Green Zone where M is 20 or more
Once you are in the green zone it means that you have sufficient money to play as you wish. This is mainly why it is called the green zone, because you can use any weapon, strategy or betting style you want. Here you must balance the way you play so that you will both protect and raise the amount of money you have in the stack. You can afford to play either tight or loose, whatever suits best your purposes.
Zone 2 - Yellow Zone where M is 11 - 19
Once you got here, it means you lost some money and you can not play any more passively. You have to get aggressive with certain hands so you can get back on track. Risks are required because this is a tournament and you are directly competing with others in order to win real money. Also, you lose the ability to play some hand that can not get you the required odds necessary for a profit. You simply have to increase your stack again so you can play freely.
Zone 3 - Orange Zone where M is 6 - 10
If you are in this zone you are beginning to find yourself in serious trouble. You now can go and bet only when you are sure you can afford it. You will mostly check and call even if you have a winning hand because you might not have the stack to do more than that. You can even try to reserve all your chips for a move that you get boosted, like an all-in move, but be careful and make this move only if you are sure of the positive exit. Avoid all marginal calls and drawing hands that can get you in trouble and can make you lose gain because you really can not take another loss.
Zone 4 - Red Zone where M is 1 - 5
If you managed to drop till here then you have no chances beside the all-in move. Because if you try and bet, even minimum, you will be obliged to play the hand and other player will try to take you out even if they can not beat the hand. When your m is less than 3 you must expect to get called by players when you go all in. So, act respectively and play if possible only the winning hand, the hand that will get you the pot after that all-in bet because that is the only chance you stand right now if you want to play further in this tournament.
Zone 5 - Dead Zone where M is less than 1
Once in the dead zone, you are probably out of the tournament or will be in a couple of minutes. You need an extreme luck to get back on track and you also need to play on a table with players that will not pay attention at all to your stack chips and therefore will not try to get you out of the game. Wait until you are first in and move in with any cards you have at that moment. And then, if you get lucky you can win something. If you do not, you'll lose knowing you tried your best.