The KReversi Handbook: Strategy Hints

Try to put pieces on the edges whenever possible. Your opponent can not outflank these pieces.
Avoid putting pieces on one of the three neighbor fields of an corner unless you are absolutly sure that your opponent will not be able to put a piece on an corner.
Sometimes it is better to offer some pieces to your opponent.
Try to put pieces on fields which prevent your opponent from moving.
Try to force your opponent to put a piece in a neighbor field of an corner.