You'll get a better seal on your muffler when using a composite gasket if you soak it in oil for a couple hours before installing it. After running the engine for a few minutes check to make sure the bolts are still tight. You might have to do this a couple times until the gasket compresses fully. Do not over torque the bolts. Use a lock washer if you can. If you over torque the bolts you'll take all the stretch out of them and they'll never stay tight.

Sometimes it's better not to use a gasket at all. Instead, clean both surfaces with alcohol or acetone and then put a very thin layer of Permatex Ultra Copper RTV (available at most auto parts stores) on both surfaces. Check tightness after the engine has warmed up.