combining stereo audio into mono

[Adam]

I picked up this little radio shack 4-way a/v switch box and I want to modify it to output mono audio with the R and L channels combined. It shouldn't be hard to do, but I've seen this done two ways. One way with just two resistors (usually a few hundred ohms each) one on the R and one on the L tied together. The other with an additional resistor (of about 10x higher value) between the combined output and ground. I'm guessing it sort of makes a voltage divider and fixes the output at a certain level. Which method works best?