There are gaskets under the valve cover that seal the spark plug holes. Items 6 and 7 on this picture.
First I would try tightening the screw on the center of the valve cover (goes thru the small hole on item 6)
If that does not work, there is an excellent DIY on replacing the whole valve cover gasket. I would do that if any of them were leaking like you described.
Here is the link:
http://www.318ti.org/forum/showthread.php?t=12007