Three things you try to do in the opening:
Other opening guidelines: Move Knights before Bishops. Don’t start an attack until ALL your pieces are ready. Don’t move up your Queen too far where your opponent’s Knights and Bishops can attack it and win tempos. The player who makes the best (and the fastest) use of his Rooks usually wins the opening!
Always think twice before making a pawn move. Since pawns cannot move backwards it is very hard to fix “pawn weaknesses”.
If you have the spatial advantage, avoid trading pieces. If your opponent has spatial advantage, trade pieces to eliminate the advantage.
2. Middle game
If you have an extra minor piece, exchange pieces to increase your advantage. If you are a minor piece down trade pawns, not pieces (there is no way to checkmate with a minor piece and a king alone).
Always play with a plan. Playing with a bad plan is a LOT better than playing with no plan whatsoever.
When you develop a plan, your men can work in harmony. For example, you might plan to attack your opponent’s king; one piece alone probably wouldn’t be able to do much, but the combined strength of several pieces makes a powerful attacking force. Another plan could be taking control of all the squares in a particular area of the board.
In any stage of the game ALWAYS realistically evaluate the position. Do not underestimate or overestimate the attacking potential of your opponent. I know it is hard to do, but that’s important. Some players overestimate the opponents attack considering that it is dangerous when it’s actually not, and starts passively defend. Some players, in opposite, completely ignore any attack of the opponent and get checkmated fast.
When there are a few pawns on the board, bishops are much stronger than knights. When there are many pawns on the board knights are stronger.
Material matters more in the endgame than in any other phase. In other words, sacing material for other forms of compensation is done a lot less frequently in the endgame.
Pawn weaknesses become more pronounced in endgames. For example, you can get away with doubled pawns in the opening and middlegame, but in the endgame they are almost always a liability.
A key endgame strategy is the principle of two weaknesses. Whereas one weakness is usually defendable, good players set out to create a secondary weakness in their opponent’s position with the idea of stretching the defense to its breaking point.
Take your time/do not rush. One mistake/inaccuracy in the endgame is often all it takes to turn a win into a draw, or a draw into a loss.
Have the right attitude. Learn to like/enjoy playing endgames and you ll do much better than those players who dread them or find them boring.
I’ve never seen anyone win a game by resigning. On the other hand, I’ve seen hundreds of stalemates when huge armies couldn’t finish off a lone king. Are you positive that your opponent can win the position on the board? If not, then play on!