Proper bypass capacitors on DVCC, AVCC, and reference pins if used.
Avoid routing any high-frequency signal close to an analog signal line. For example, keep digital switching signals such as PWM or JTAG signals away from the oscillator circuit.
See Circuit Board Layout Techniques for a detailed discussion of PCB layout considerations. This document is written primarily about op amps, but the guidelines are generally applicable for all mixed-signal applications.