You may not want to invest lots of cash buying stock for a corporation that is positioned on the edge of the gameboard and away from other corporations.
You may want to invest in corporations more likely to merge so that you receive a majority or minority stockholder's bonus.
Diversify! Receiving a majority or minority stockholder bonus is an excellent way to make more money. Keep an eye on the smaller active corporations.
Try to have the most stock or the second-most stock in these corporations.
Toward the end of the game, you may also want to own stock in the larger corporations, again to capitalize on potential bonuses.
When a smaller corporation in which you have stock is taken over, sometimes it is wise to trade in two for one if you think you have a chance of getting a majority stockholder's bonus in the larger corporation.