I haven't played risk too many times so I didn't develop good strategies. But the 3 times I played with my nephew's (10 years old) whoever had North America won (three players).
They both teamed up against me and I ended up getting sandwiched in Europe and beaten. I tried getting as much land as possible, to get as much pieces each turn as possible. Which worked pretty ok since my nephew that had Australia didn't really want to start any wars, just build up his army.
But I think my mistake was in the beginning of the game I tried to break up North America and failed to do so. Resulting in losing a lot of pieces for no reason, army spread too thin, and my nephew strengthening my only access point to N America with his bonus pieces. And going down through Africa wasn't feasible due to an army too thin to make it there before he strengthened the other access points.