I've always used the method Ken Mosher suggested, I believe.
- Paint the gaskets with a high-temp silver paint and let dry
- Install the gaskets in the covers after coating with a very fine bead of O2 sensor safe high-temp sealant, (Note: gaskets may require a bit of trimming in the OEM valve covers) which can be found in grey/silver.
- Let dry overnight
- Install on engine - no sealant required. Do not overtorque.
http://www.gnttype.org/techarea/engine/valvecovers.html