What should be kept away from a boat compass to ensure accuracy?

Magnets and iron. A compass has a needle that has a magnatized end. If there are outside objects near then it will give a false reading. I have placed a knofe in the console before and had my compass read 20 degrees off. If this had gone undetected the results could have been the difference between running aground or getting lost or getting where I wanted to go. Car keys will do the same thing.