Now I'm not sure what the current carrying capacity of the board itself is, but could you perhaps buy another power supply to replace the one that is supplied with the arduino board, one that can output a larger current but maintains the same voltage of the original power supply.
If this isn't possible, what would you do to connect the ground from the servo to the secondary power supply if the power supply is one of the types that is double insulated and has no earth pin to plug in the wall socket? (Yeah, we have these in Australia). Would pinching the earth from the arduino's power supply work, since it's all of the same continuity anyway?
I'm looking to do a similar thing with multiple servos in the future. I dont know what load it will be, so overkill would probably be a good idea in my case.