I think the biggest problem is caused by over tightening the 'hold-down-nuts'.
If the cork has the adhesive on it, clean the ally cover very well and stick the cork to it, or apply cork gasket adhesive first. Be very carefull lowering the cover onto the head so as to not disslodge the cork. tighten the nuts EVENLY and 'just' over finger tight. Check them in a week or so and you may want to re-tighten them if the are easily un-done with your fingers. I find that eventually the cork changes shape of it's own accord and no longer wants to remain in a shape that seals the cover. Using light tension on the nuts seems to extend the usable life of the cork gasket. I prefer to toss the cork and simply use a 1/8th inch dia bead of silicone gasket goop. Let it set a bit, till it skins over, then install the cover with very loose nuts.
Hopefully others will come on here and slam me for my comments and tell me a fool proof method of installation. So far I've been most successful using one of the two methods above.
Yeah, this doesn't have to be super tight as the function of the valve cover here is to keep crap out of the engine internals and keep oil from shooting all over the engine bay.
I used something called "Indian Head Gasket Shellac" to attach the cork gasket to the cover itself. The other side shouldn't be glued down as you'll need to remove the valve cover to adjust the valves occasionally...