I think the trick to keep the gasket from sliding out is to make sure both gasket surfaces (head and valve cover) are free of any oil or gasket sealer that would cause the gasket to slip. In other words, the surfaces need to be dry. Torquing the fasteners also needs to be done evenly in a crisscross fashion, gradually increasing the torque to spec. Fully torquing down a single bolt to spec, then progressing to the next, etc etc will leave you with a mess.
I've had my vc cover off multiple times without changing gaskets with no leaks. The only sealer I have used is a dab in the corners to hold it in place but even that isn't needed if the gasket is flattened because it has a shoulder to catch the metal.
The only time my vc cover gasket leaked was during the first 500 kms. All the bolts were loose and needed to be snugged up. Since then, no issues.