Ok this thread is full of stupid responses.

Shams idea of lock washers and Nemos gasket thought are the two most logical....the rest are plain moronic.

Like Nemo said check the gasket around the valve cover and make sure it is located correctly in the slot. Also while it is off check the rubber grommets located at each of the valve cover bolts, if they wear out they can cause play which ill allow the cover to loosen up easily. If neither of those is the case then try the lock washers that Sham suggested....this is kind of a last effort though