Play Large Pieces First
In general, larger pieces are difficult to use later in the game. Because of this, it is a good idea to try and play larger pieces early in the game. Playing five and six-sized blocks early allows you to get them out of your pile, lets you advance towards your opponent so that you can get in his way and can make it difficult for your opponent to play pieces near your starting area. Much like using early pawns in chess to control space, this strategy gives you room to build later in the game.
Build Towards the Center
The object of Blokus is to have the fewest squares left when you can no longer place a piece. Building towards the center helps by giving you more options and reducing the legal plays your opponent can make. Placing blocks near the center makes it more difficult for your opponents to play his larger blocks. By placing as many pieces as possible in the center, you can make it hard for him to lay blocks and make it all but impossible for him to encroach on your starting territory.
Unreachable Walls
Players must always lay a block so that it touches only the corner of their other pieces. Because of this, it can be difficult to wall an opponent out. Since your pieces can only touch at corners, opponents can generally squeeze through the gaps in your walls. To keep opponents out of an area that you want to build in, you must consider how to set up these walls carefully. Work to set up walls so that your opponent has to either use a particular piece to get through them, or choose to build around in another direction.
Plan Ahead
Blokus is a strategy board game. There are no dice, no cards and no drawing randomly from a bag. This allows you to plan for the future. Watch what pieces your opponents have left and decide what moves they are likely to make. Decide how best to either work around them or block their plans. Watch which blocks your opponents have left and plan your turns accordingly. This is especially important near the end of the game when everyone has limited pieces and is looking to place as many blocks as they can.