We can all manipulate our SAC rates by changing our breathing pattern, but we really should resist the temptation. Breathing through a reg underwater is not natural, but we should try to do it as naturally as possible.
I find that the more I dive, the less I think about my breathing. When I am teaching and working underwater, I breathe normally, naturally and as required. With challenging students my SAC goes way up, with easy students or just diving for fun in easy conditions' the SAC goes way down. As long as I plan ahead and carry appropriate back gas, I really don't worry about it or try to change it.
Solid buoyancy, perfect trim, moving slowly, effective finning, a calm demeanor, and relaxed rhythmic breathing, make the SAC rate level off where it is supposed to be.
IMO experienced divers lower their SAC rates because they are more comfortable underwater, more relaxed, and their bodies are demanding less O2. At some point, no matter how relaxed and efficient, we breath what we breath and the SAC fits the individual.
As TS&M suggests, the best way to prolong a dive or to increase the safety margin with a planned minimum reserve, is to use a higher volume tank. It worries me that some suggestions on Scubaboard could encourage risky diving habits. Improper breathing and the resulting CO2 buildup can significantly increase the risk of DCS.
Those seeking advice should research the advisor's profiles, previous posts and even then, draw a bell curve to see where the concensus lies. Look to a trusted outside source. DAN has some excellent articles on their website.
Please be careful.