The core rules
A separate fund you can afford to lose; a flat stake of 1–2% of it per bet (up to 5% for aggressive plans); no increase after losses. At 2% stakes a 10-bet losing run — which happens to everyone — costs about a fifth of the bankroll, not the whole of it.
Why flat beats chasing
Progressive schemes («double after a loss») do not change the expected value of a single bet — the odds contain the same margin — but they concentrate the risk of ruin into one bad evening. Flat staking spreads variance; chasing multiplies it.