The best strategy I've found is that you first take North america.
Each border country only has 1 country next to it and there are only
three border country's. Once done with this eather take over south
america or europe, the one which is weakest. After that take africa.
by then you will have anough armies coming in that it is almost
inpossible to beat. I've tried this strategy many times on my computer
version of Risk and I almost never have lost. When I have it was
because I never was able to take North America.