Bite the bullet and buy a new set of valve cover gaskets. Most cork valve cover gaskets usually cost less than $20 and are fairly easy to install on many engines. You may have to disconnect and remove some plumbing or other accessories to get to the valve covers, but on many engines the job is usually within the capabilities of a do-it-yourselfer. If the valve covers are buried or access is difficult, then let a professional replace the gaskets for you.
Tightening the valve cover bolts or screws will rarely stop an oil leak because the gasket is usually cracked, crushed or has lost its natural elasticity. Cork gaskets only last about four to six years before they age harden, become brittle and start to leak. Molded silicone rubber gaskets, on the other hand, (which are used on many late model domestic and import engines) often last the life of the engine. But molded rubber gaskets are a lot more expensive than die cut cork gaskets. That's why cork gaskets have long been used by the vehicle manufacturers.
Some engines do not have gaskets, but instead use a rubbery-glue called "RTV silicone sealer (the "RTV" stands for Room Temperature Vulcanizing). If this is the case, you can remove the valve cover, scrape off all the old RTV, and either apply a fresh bead of RTV silicone sealer to the valve cover flange or head mating surface or install a conventional gasket.
CAUTION: Do not let any pieces of rubber or debris fall into the engine. Also, if you decide to use RTV sealer and your engine has an oxygen sensor (which almost all 1981 and later engines do), make sure the RTV sealer is the "low volatile" variety that is approved for use with oxygen sensors. Some types of RTV give off silicone vapor that can be sucked through the crankcase and contaminate the oxygen sensor.