Zener Diode for overvoltage protection
You could consider a Transient Voltage Suppressor (TVS). They are basically diode arrangements that are intended to protect circuits like yours from over/under voltages. Since they are designed to work with microcontrollers and other devices for this purpose, you can choose the voltage range you're using when you buy them (e.g. designed for 3.3V and goes into a full clamp at 3.5V).

Operational voltage ranges and tolerances for these devices are typically better than straight-up zener diodes.

Negative protection is included in the package as well. They work very well for ESD protection. There are a lot of them out there, but here is one example:

http://www.semtech.com/images/datasheet/uclamp3301d.pdf