The above methods will slow the electrolytic errosion of the electrodes, but will not prevent it. To prevent it completely, without plating with expensive metals lilke gold, use a biphasic measurement as follows:
Connect the sensor between two port pins and have a pull-up resistor to Vcc from each. Then perform two port measurements: first by setting pin A as output and drive it low, use _delay_us() to define a measurement delay, read pin B then restore it as input. Next, set pin B as output and drive it low, delay for exactly the same time, read pin A then restore it as input. Use one pin reading to read the sensor (ignore the other).
If both pin measurements are equal, then there is no net charge transfer between the electrodes, and no net electrolytic erosion.