I need to sample an analog voltage from a sensor (0-12v) in an environment where large voltage spikes may be induced in the transmission line, what's the best (simple and cheap) method of protecting the ADC input pin of a PICAXE?

FYI: I'm thinking of using a simple voltage divider to reduce voltage range of the sensor, and a TVS to protect the ADC input.

A simple pair of diodes is all you need. One from the ADC input to $V_{CC}$, anode on $V_{CC}$. One from the ADC input to GND, cathode to GND.
If $V_{in}$ goes above $V_{CC}$ then the excess will go through the upper diode. If it falls below GND then it will flow through the lower diode. The input to the ADC will be just the range between $V_{CC}$ and GND.
• Relying on the integral diodes is generally a bad idea. They're there for ESD protection, not regular surge use. Also, you really should use Schottky diodes for the protection network, because their $V_{TH}$ is guaranteed to be lower then the IC's integrated diodes. Lastly, this system is reliant on your system having enough capacitance across the rails to absorb the energy from the spikes. If there is too little capacitance, the spikes will cause $V_{CC}$ to increase. – Connor Wolf Nov 26 '11 at 11:12