I would check vayama.com on a regular basis. We just found pretty reasonable tickets into Beijing and out of Moscow, and they have a little search button for deals.
For our trip to Mendoza we flew into Buenos Aires and then took a 13 hour bus ride from BA to Mendoza. It was, by far, one of the best bus trips we've ever been on. You can "upgrade" and get seats that fold into a bed (complete with blanket and pillow--very comfy), so we both got our usual 8 hours of sleep, too. After hopping on the bus, eating dinner (it comes with the upgrade), sharing a bottle of wine, we both fell asleep and --presto-- woke up in Mendoza. For the return trip we flew, and we really wished we'd taken the bus on the return trip, too, as it was much more relaxing. I think we paid about US 50 for the one way bus ticket. Totally worth it. You can easily book tickets at the big bus station in Buenos Aires a day before your trip to Mendoza. PM me for more specifics if you are interested.