2. Position. Acting last is the single largest structural edge in poker. In position you see your opponent's action before committing, control the size of the pot, and can realise equity with marginal hands that would be unplayable out of position. Play more hands from late position, fewer from early.
3. Bankroll management. Cash: 30 to 50 buy-ins for the stake. Sit & Go's: 50 to 100. MTTs: 100 to 300. Never buy in for more than 5% of your bankroll in a tournament, and drop down a level after a defined loss threshold rather than trying to win it back at the same limit.
4. Decision-making at the table. Compare the price you're being offered against the chance your hand improves or is already best. Count outs, convert to rough equity, compare to pot odds. Fold when the maths says fold, even when it feels weak. And when nobody has bet, taking the free check is a decision too, not a non-decision.
For house poker: the strategy set is much smaller but still worth learning. In Casino Hold'em, raise with any pair or better and with reasonable draws on the flop, and fold only genuinely hopeless holdings. In Caribbean Stud, the standard rule is to Call with A-K or better and fold everything else. In Three Card Poker, Play with Q-6-4 or better. In video poker, use the correct hold strategy chart for your specific paytable, because deviating from it can cost more than a full percentage point of return.
Track everything. Record buy-ins, cash-outs, hands played and format. Without a record you cannot tell a downswing from a leak, and you will keep repeating whichever one it is.