Blokus is a strategy board game that can be played with two, three or four players. The object of Blokus is to play all of your pieces (or as many as possible) on the game board. Game pieces can be played anywhere as long as they are touching at least one corner of another piece of your color and hey are not otherwise touching any pieces of your color.
Hem your opponent in. This is a general sort of strategy because, as the game progresses, it will limit your opponent's moves, preventing him from taking a turn and allowing you to win the game. The complementary part of this strategy is to leave yourself with as much space as possible in which to make moves.
Play the biggest pieces first. The larger pieces get more and more difficult to play as the game progresses and there is less space on the board, so it is wise to play them first.
Get to the center of the board as quickly as possible. This strategy combines aspects of Step 1 and 2. This gives you a strong base with which to hem in your opponent and is aided by playing your larger pieces first. So, when playing your larger pieces, situate them so that they create a bridge that goes as directly to the center of the board as possible.
Protect "your" space. Blocking off your opponent only works if she cannot move past your own line of blocks. As a result, if you are trying to hem in your opponent, be wary of areas where he could slip through a line of blocks.
Keep an eye on your opponents remaining pieces. This strategy works best toward the end of a game, as both the number of pieces and possible moves goes down. Thinking abut what pieces your opponent has left (either by looking at his unplayed pieces or by deducing it from looking at what pieces she has played) can help you guess what your opponent's next move(s) will be and give you a chance to thwart her strategy.