1 Develop your pieces

Get your pieces out quickly and efficiently. Don't go pawn hunting. A player with 5 developed pieces vs a player with 3 developed pieces has a distinct advantage.

2 Get your king to safety

Castling is important. Don't start an attack with your king in the center.

3 Fight for control of the center

Whether it's by occupying the center with pawns or attacking the center via pieces, control of the center is vital because it will give you more space to operate.

4 Pick an opening for White and two for Black and learn them thoroughly.

Don't play mulitiple openings. Learn a good solid opening for white (Colle, for example) and two defenses for Black, one each against 1 d4 and 1 e4. Try to find a similar defense for the two as this will help you understand the patterns common with those openings. If you can get out of the opening with only a minimal disadvantage as Black and a minimal advantage with White you will have better chances in the middlegame.

5 Study basic chess endings.

A good knowledge of basic chess endings will many times save what seemed to be a lost game. Many players are not good as chess endings because they are boring to study. This is where you can gain an advantage. Remember one thing in rook endgames--keep your rooks active!

6 Develop a good positional understanding.

Understand pawn structures, how to attack pawn chains, attack weak squares, dark square/light square strategies, transition to favorable endgame positions...

7 Secure outposts for your pieces

Good knight outposts can determine the outcome of a game. For example, a white knight on e6 many times leads to a won position. Rooks on open or half open files. Bishops on opposite color of pawns.

8 Avoid creating weaknesses.

Backward pawns on half open files, too many pawn islands, weakened areas around kings, good/bad bishop diagonals, knights on the rim vs in center. Learn this.