Observe and Assess
It may seem obvious, but paying attention to your opponent's moves will help you make yours. Try to figure out what he is trying to accomplish by making that move. Conversely, when you start thinking about your own move, consider the consequences. Have a plan and know what will result from your move.
King Safety
You must always be aware of your king. You want to keep your king safe. Castling is a technique that brings your rook into the forefront while protecting your king. Some people become so focused on attacking their opponent's king that they forget to protect their own.
Piece Value
It is inevitable that you will lose some pieces during a game of chess. The key is losing pieces that have the least value. Pawns have the lowest value on the board followed by knights and bishops. Rooks and queens have more flexibility on the game board and you should protect them whenever possible.
Endgame
Maintain some foresight to the endgame. Some pieces, like a bishop, tend to have more power at the end of the game. Pawns also gain more value. You may want to move your king up into attack mode if the situation presents itself. Think every move through so you are set up well for the endgame.