If we charge an object made of insulating material, the charges on it would leak to the medium as the time passes, due to the potential difference. I would like to know if there is a way to prevent leakage and conserve the charges on that object. I think, if we cover the object with an insulator which has higher volume resistivity than the insulator the object is made of, we would be able to make most of the charges stay in place. However, I am not sure if the charge flow is related with the electrical resistivity of materials. Is there a law which simply states that the charge flow could be prevented with good insulating although there is a potential difference? I know that the current in amperes is directly proportional to the conductivity of the material used in a circuit but would there be a way to create nearly no current at all?
When you charge an insulator, you typically put charge just on the outside. The insulator presumably needs to be connected to the "earth" in some way, and it is important that you use a good insulator for that mechanical connection.
You will find that the main leakage mechanism is usually moisture in the air. As long as the charged surface of the insulator is in contact with air, it will discharge (more quickly on humid days - this is why your clothes get "more static" in winter, since the humidity in the air tends to be lower). So if you can protect your insulator from moist air (maybe by putting it inside an evacuated chamber - or at least by surrounding it with another container and getting rid of the humidity) it will help a lot.
Sometimes you can help by including guard electrodes - these are electrodes that are designed to create zero electric field at some point along the conduction path, and which will thus prevent leakage.
More about charge loss on insulators can be found in this earlier answer