(Or, minmaxing chess for the 1-3 times I play per year)
White: Vienna Game/Gambit
Black: Caro-Kann Defence
Don't place pieces where they're undefended or will be vulnerable while you're setting up their defense
Bring rooks into backline centers
Exchange pawns to open up center lines
If moving pieces to opponent's side of the board (4 lines their side), don't lose material (consider number of ways a piece is attacked/defended)
Look for (any and all) checks, captures, and attacks (forks, pins, skewers, etc), in that order
Middlegame: start making a plan, moving pieces into strategic positions and looking for targets (removing enemy materiel, corralling king)
- Start calculating here
Punishing early queen shots: DON'T PLAY g6, look at all the possible threats and defend with a knight (not actually that hard); you can play g6 afterwards; alternately, counterattack by hitting the queen (forking the king if possible); occupy the middle to take advantage of them not developing
KISS: don't try to play like a master if you're a novice
Against a nonplayer, hyperaggression works to my advantage (it throws them off); against a player, a high-variance strategy gives me an outside chance of winning via bullshit
Look at ALL THE PIECES ON THE BOARD, SOME OF THEM ARE IN PLAY AND YOU DON'T REALIZE IT
Castle early, develop knights and bishops, get rooks into the middle, play on your side, not the opponents
King needs to be active in the endgame (it can herd, since you can't take it)
Look for unprotected pieces; if they're attacking and undefended, you can just take, ez
Don't tunnel vision, always ask 'what does my opponent's move accomplish, and what's his best move after I move?'
Don't start calculating hard til middlegame
Stay on your side until the middlegame, developing an attack
Trading literally everything (sometimes you have to take it on the chin to avoid screwing yourself over)
Making moves that create simple threats and take your pieces out of position
Not thinking about the opponent's goals