Risk is my all-time favorite game... But really, the best strategy is to be able to talk faster/smoother than the rest of your friends! I normally play with the same group of guys, and really it always comes down to me and one other friend in particular who are always running off at the mouth trying to con everyone into doing what you want them to do. Ideally you want a nice position, but you don't want to come off insanely powerful, because then everyone will be more likely to team you. I always pull the, "Hey, look you think I'm bad, but Joe over here is steamrolling everything and he's gonna own us all if blah blah blah" and try to kick the attention away from me
But as for more concrete strats... I find Europe is nice, but a bitch to hold since you're right in the middle. Asia is all but impossible so don't try. People who take Australia early on seem to win a lot, because they are safe and can either sneak THROUGH Asia (not take the whole thing) and into N.A or Africa. However we usually play with extra passages into Aussi to stop that, like from Madagascar or S.A.
One really helpful trick is that if you have armies massing on your borders next to your own armies, here is what you do: On your turn, attack the living bejesus out of the army. But whatever you do, do not take the country! Just beat him down to 1 or 2 guys. Ideally you will still have a nice chunk of army in your territory and now his border is weak. For his next turn, he will have to put his reinforcements back into that spot...
Now the fun part is, since you left it, all his other armies can't steamroll back into it from other countries and then continue into your territory. It is kind of hard to explain and visualize, but if you know what I'm saying, it makes sense I guess
I think of it as draining the resavoir, not breaking the dam!