After reading a recommendation in this forum over 8 years ago, I have used weatherstrip adhesive to attach the cork gasket to the valve cover (and side cover, and oil pan). I put no adhesive on the other side of the cork gasket. Then I attach the parts as described by Hotrod Lincoln - do not over tighten. I have not had a leak and I have had all parts off (and then put back on) multiple times.
Also, it is quite common for these sheet metal parts to be deformed near the bolt/screw hole due to over-tightening in the past. The screw holes in the sheet metal should be punched back out, if they look like they are deformed inward. Otherwise, you will have leaks.
