♠
Playing too many hands
+
Strong players fold most of what they're dealt and wait for spots where the odds actually favour them.
♥
Ignoring position
+
Acting last means seeing everyone else's decision before making yours. That's worth more than most players realise.
♦
Chasing losses
+
Bigger bets trying to win back a bad session usually lead to a worse one. Knowing when to walk away is a skill too.
♣
Never folding after the flop
+
Chips already in the pot don't obligate you to add more. Treat every new card as new information, not a reason to justify it.