I've noticed when I start really sucking air, breathing through my nose (though sometimes its easier said than done since it can get pretty stuffed up during a long ride) tends to regulate my breathing a little bit better, and I generally calm down somewhat and feel less tired. Don't know if that's a physical reaction or just psychological, but it works for me. Or, if I take a really deep breath it seems to have the same effect, so I guess some of that advice is worth noting.