After a change of the valve cover gasket didn't fix a rather persistent leak (than managed to cover my entire engine in oil) I took my car to a mechanic. They called up saying that they have pinpointed the problem to one corner of the cover that they say seems to have warped slightly. Now this is a thick plastic cover, I am just wondering, is this warping common or is it indicating a larger problem? Is there anything I can do to prevent this in the future?
This would depend on the type of car you have. The majority of valve covers i've come across are aluminum.
You can try some RTV sealant or Hondabond sealant (pretty popular solution for this) in place of a valve cover gasket. The sealant should mold itself around all the gaps and irregularities in your valve cover, making for a better seal.
If that fails, see if an aftermarket aluminum valve cover is available for your car. They make them in colored/polished/chrome finishes for all kinds of cars - chances are they may have one for yours.
A few year ago, I remember Stacey David (when he was hosting the "Trucks" show) commenting that he thought the plastic valve covers on the old Jeep Wrangler must have been designed from the factory to leak because he had never seen one that didn't. That doesn't sound promising. Maybe you can find some aluminum or steel replacement valve covers from ebay, car-part.com, or from rock auto. What kind of car is it? That will help.
EDIT: Here is a picture of the new valve cover gasket that I think will be needed on your car. I don't see any aftermarket valve covers (of any kind) for this car, so I guess plastic it is.
Here is a LINK that describes the process of replacing it. It all sounds pretty accurate, except for the part where they tell you to speak with a Saturn mechanic... doh!