Im not 100% sure on how to retard timing, but I am pretty sure the plug temperature does make a bit of a difference, but more in the engine temperature sense - Say you are running a #6 and it's nice at 105 degrees. You switch to a #7, it should take your temperature down because the plug is "extracting" heat from your engine headbutton faster than the #6. But I believe it will also delay the firing time as well.

Changing your head gap will change timing also but I would only run on the recommended stock settings or larger up to a 0.2mm limit.
I've heard that a few people ran their engine on stock shimming, then increased it by 0.1mm and there was a better and noticeable difference in power and fuel consumption, but that may have just been the "track factor" because having a larger gap should give better performance on the top end.

Im sure more of the engine guru's here will chip in and correct me if I'm wrong - I'm not completely sure, but that is what I understand.