The other thing to remember regarding over tightening is to make sure the cover is not pushed down or bent around the fastening holes from seriously over tightening it. 

Whenever this topic comes around I offer the following advice.  The factory valve cover is easily distorted.  Gluing/bonding the gasket to the cover doesn't always address the warp.  Following the steps below does.

Remove the valve cover. 
Scrape the old gasket and all traces of old sealant from the valve cover.
Likewise, remove any sealant and gasket pieces from the top of the cylinder head.
Wipe down the underside of the valve cover with brake or carb cleaner to get all the old lube off.
Smear the top of the cylinder head with a thin layer of grease.
Apply RTV to the underside of the valve cover, then gently lay the gasket onto the RTV.
Flip the valve cover over and place it on the cylinder head and gently push it down.
Tighten the valve cover fasteners ONLY finger tight... no tighter.   Wait overnight.
The next day, after the RTV has cured, tighten the valve cover fasteners to the specified torque.

Why go to all this trouble?  The grease on the top of the cylinder head allows the gasket to conform to the flat surface of the head.  The RTV both bonds the gasket to the cover AND fills the gaps that may be present if the valve cover is distorted.  Once the RTV has set you can remove and replace the valve cover multiple times before the gasket has to be replaced. 

3M  part # 08008. black super weatherstrip and gasket adhesive

use it on the valve cover side, put this product on the gasket and then stick it to the valve cover

let the adhesive air dry on the gasket for a minute before you stick it to the valve cover

 

As others stated just put some sealer on the one side.
I store my spare ones over the top of an old valve cover, when i need one they are the correct shape.

Long ago, I bought a bunch of cork valve cover gaskets. When I needed one, I found they all shrunk.  Rather than wait for shipping from our host, I bought some sheet / rubber sheet gasket from the local auto parts supplier and made my own. Took more time but hasn't given me any problems.

+1 to using an adhesive (not a sealant that may take a long time to cure) on the rocker cover. Let it dry/cure before trying to install the cover. I have found that cork gaskets tend to bow in on the long sides. I use clothes pins to hold the sides in line until the glue sticks. Really old-school is/was to use gasket shellac.
When you do put the rocker cover on, the bolts should be just slightly more than finger tight or the cork distorts. You only need enough pressure to get the gasket to make full contact with the cylinder head.

Rubber contact cement to the valve cover and that ONE side of the gasket and it cannot move, you may need a new gasket at this point. Try not to get it on the gasket edge, the oil is meant to swell the gasket to create the seal on cork types. if you have a sheet metal valve cover make sure it is flat in the gasket area, they often get bent at the bolts. Just my way.

I cannot get the cork valve cover gasket to stay in place ..after tightening down it wants to move to the inside of the cover lip and not seal..any suggestions?  yes I used a good adhesive/sealant on the cover