A very very simple tip that even a 12 year old can do. If your toilets water closet, some prefers to call it a cistern, is overflowing, all you have to do is to tighten the level adjuster inside the cistern. Tightening the screw (shown here) will shut of the float valve faster, so the water level in the closets will be lessor. Loosening the screw, will increase the water level as the valve takes longer to shut of the water. Adjust it to suit your needs.
just open the covers of the cistern and you will know what I meant. However this tip is only for those these low quality cisterns. Those high end one looks much more nicer but it is very hard to maintain, spares are hard to come by and expensive.