One thing I like to do when making a voltage divider is put a trimpot between the two resistors and adjust for the best reading possible. Connect the wiper pin on the trimpot to the arduino and adjust the trimpot until the Arduino reads correctly.
The trimpot should be the lowest value possible that covers the range you need, to allow easy adjustment. In the example peter247 gave with 100K and 47K, I would suggest either a 1K or 2.5K trimpot. You may need a larger trimpot if the resistors are off by much.