You can use a MOSFET as a switch in order to connect/disconnect the battery to/from the ADC input. The MOSFET state is controlled by a uC pin. The battery is connected to the ADC only for a short time, when the voltage is being measured. If, for example, you need a sample every minute, and it takes 1 ms to take it, the reduction in battery life will be negligible.

I have used this approach in the remote sensors of a wireless wheather station. It works fine. Using this technique you can even drain a suitable (nominal) current from the battery in order to get a more realistic information about the battery charge.

Alfredo del Rio.