dc42:
If the voltage source really does have a very high source resistance, then you can increase the values of the resistors in the voltage divider, provided that you take appropriate precautions when using the ADC to read the voltage, and that you don't need to measure a rapidly-changing voltage. I've used two 4.7M resistors in one of my designs to monitor the voltage of the 9V battery powering a circuit that had no on/off switch. However, it seems strange to me that at voltage or 9.5V should have such a high source resistance.
Well, it seems you get the prize... if there was one. I put a 4.7M and a 10M resistors in there and the voltage drop is very minimal. All that's left is to see if I'm getting correct values with the arduino calculated properly.