Strategy & improvement
Losing a winning position because your flag fell is one of the most painful ways to lose. Clock management is a skill you can train like any other.
Not all moves deserve equal thought. Play known opening moves and obvious recaptures quickly; save your time for critical moments — sharp tactics, irreversible decisions, and complex middlegames.
A position is critical when the character of the game could change: a possible sacrifice, a pawn break, a forced sequence, or a choice between plans. These are worth a long think; quiet manoeuvring moves usually aren't.
Roughly divide your time over the expected number of moves and check yourself against it. If you've used half your clock by move 15, you're thinking too long on non-critical moves.
Once you're low on time you blunder more, which costs more time worrying. Build a small buffer early so you're never forced to make 10 moves in 30 seconds in a sharp position.
Time controls with an increment (e.g. 5+3) reward steady play and make flag-falls far less likely. If you struggle with the clock, play increment time controls while you build the habit. Try a game free and watch where your time goes.
Play obvious moves quickly and save your time for critical moments. Keep a rough time budget, build a buffer early, and play time controls with an increment so a single long think doesn't cost you the game.
A point where the game's character could change — a possible sacrifice, a pawn break, a forced sequence, or a choice between plans. These deserve a long think; quiet moves do not.