Kind of overkill, but I use a cork gasket and lightly silicon both sides, spray out the bolt holes with brake cleaner, a drop of Loc Tite then just snug them, let it cure over night. If it´s done right, it will rarley ever leak, even with missing bolts and a shrunken gasket. Seriously cleaning oil from all bonding survices is necessary.
Removing the valve cover again, requires a little more cleanup, but is in the long run worth it. I usually pry up a little and cut through the gasket with a carpet knife, when removing the valve cover. Less chance of bending it, than trying to pry it off.
Same technique with the pan gasket.
A straight cork gasket will also work just fine, without the silicon. But if you don´t remove/replace them occasionally, they will get hard and shrink.