To make sure a downswing never endangers your poker career, you need to manage your bankroll well and should never invest too much money into a tournament. This is called bankroll management.
Your bankroll is your capital, which needs to be invested wisely, so that you can progress in the game. You want to win a lot, when you win, but not be hit too hard by any losses. This can be done if you keep to the following rule:
The 100 Buy-In Rule
Experience has shown that 100 buy-ins accommodate your desire to have high winning chances and a low risk of loss in the best possible way. Don't play tournaments for which you have not made 100 buy-ins.
In other words:
Never invest more than about 1% of your bankroll in a tournament.
This rule accordingly shows you a precise approach for when you happen to move up or down a limit.