The only effective way to improve and play chess well is to play the game. Studying openings and strategies is helpful, but it only gets a player so far. When playing a game, consider each move carefully -- not just your moves, but your opponent's as well.
After each move, consider why the move was made. Not all moves are made to threaten a piece directly. Instead, some act to cover certain squares or to open up movement.
Before moving, look at the square in question. Consider whether it is under threat or if it can enable you to threaten the opponent's squares.
Try to maintain control of the center of the board. From the center, pieces can project their power more effectively than they can from the edges. When you are developing your pieces to control the chessboard, don't discount pawns. Even though pawns are individually weak, they can form a wall that is difficult to breach without loss. Also, keep a few pawns in their home squares; pawns cannot move backward, and you may need protection later.
Also, don't assume that you have the advantage just because you have more pieces. A skilled player can create forks, or situations in which multiple pieces are threatened by one piece, quickly removing any numerical advantage. Also, poor development can render your pieces unable to maneuver.