Tactical Ploys - The Fork
In the middlegame, the board is congested, which opens up opportunities to threaten your opponent's pieces and try to force him into losing material. A classic way of doing this is a "fork." This happens when your opponent has erred positionally, allowing you to threaten two of his pieces with one move. The knight, which has especially good reach from the middle of the board, is the piece which most commonly delivers a fork, but any piece can do so. This tactic can be especially devastating if you are able to fork two pieces with a pawn, as the opponent will only be able to capture a pawn for the loss of a major piece, giving you a clear advantage.
Tactical Ploys - The Pin
If the opponent's king is placed poorly, you might be able to "pin" one of his other pieces to it. A piece pinned to a king is called this because it is unable to move out of the line of the pin as it would reveal check on the king by doing so. If you are able to freeze a piece in this way, you can gain material or time to make other moves and develop your position, which can be equally beneficial. A pin does not just have to threaten a king and can be decisive when it involves a queen or rook.
Tactical Ploys - Discovered Attack
A discovered attack happens when, by moving one of your pieces, you reveal a threat on the opponent by another of your pieces whose line of attack had previously been hidden. This is especially effective if the piece which reveals the attack moves to a position where it also attacks an opposing piece. When a discovered attack causes check on the king, it can be absolutely deadly as the opponent must move out of check before he can deal with any other threats you have made.
Beware of Danger
Always study the position of your opponents pieces, not just for weak spots which you can attack, but also for any attacks that she might be trying to launch. In the middlegame especially there is the danger of succumbing to a cheap checkmate, especially when the queen has joined the attack. Ensure that your king is properly defended before going on the offensive -- by the time you are into the middlegame, your king should be "castled," normally on the king's side. Castling makes it less likely that you will fall victim to tactical moves such as pins and discovered attacks. Also, when launching your tactical ploys, you must remember to ensure that the piece you are using to attack is not at threat of being captured itself ("hanging").