Try not to play at random. Such game practice will most certainly result in loss.
Construct long chains of lines over the gameboard. Once one such chain is complete start to build another one going parallel to the first.
Try not to build contraptions where only one line remains undone. Your opponent will almost certainly take an advantage of it.
If you are to let the opponent get a square or more, try to select the combination resulting in the least damage.
Try to push your opponent into the situation where he/she(it) will have no choice but to let you get the square.
If you are to get a chance to start completing a whole chain of squares, remember that by leaving two squares at the end of such chain incomplete you are pushing your opponent into completing them, thus giving you the opportunity to continue onto yet another chain. Such a situation is called the double-cross.