Control the centre before you attack the edges
Pieces are simply more useful near the centre of the board, where they influence more squares. Early moves that claim central space with a pawn, or prepare to, tend to age far better than flank expansions launched before development is complete.
Develop with a purpose, not just a move count
"Develop your pieces" is often taught as a checklist — knights before bishops, castle by move ten — but the real principle is that each developing move should improve a piece's influence on the position. A knight developed to the rim of the board technically counts as "developed" but rarely does much work.
Don't move the same piece twice without a reason
Every extra tempo spent repositioning a piece in the opening is a tempo your opponent gets to develop or claim space. It's sometimes necessary — retreating from an attack, for instance — but it should be a deliberate trade-off, not a habit.
King safety isn't optional
Delaying castling to chase a small material or positional gain is one of the most common ways club players lose quickly. Unless there's a concrete tactical reason to wait, get the king to safety before opening the position further.
Know the plan behind your opening, not just the moves
Memorised lines run out eventually, often exactly when the position gets interesting. Understanding why an opening's typical pawn structure favours a certain plan — a minority attack, a central break, a kingside pawn storm — will serve you long after your memorised moves end.