Teflon tape should most definately be used on the threads of your fuel petcock when you screw it into the gas tank. This will most definately help to seal the threads and most probably stop the leak your describing!
Some double gasket and that helps. It depends if it's leaking at the joint or if it leaks around the lever itself. Mine would leak when shut off in one direction but not the other. I got a nice brass valve from Lucky Early Bird on eBay that doesn't leak. Took it off the chinee tank when I sold it. Went to Andy's Parts and got a bushing adapter to fit the valve to a Paugco Porkster tank that's going on my new build.
If it just continues then replacement is the only option. If the fuel drops on the motor you're rideing potential fireball
However, I would recommend throwing that piece of shoddy chinese engineering out and make a better petcock with either a ball valve or a brass spout and inline petcock from the hardware store.