Hi,
first of all, you have to identify the frequencies which your noise acts at.
Second, design both analog and digital grounds as large as possible and connect one to another thru a single point only with a trace that could be 6mm large (rule of thumb). Keep both ground on the same layer. For example, in 4-layer applications, put on 2nd layer. On 1st layer, put signals (analog upon analog ground and digital upon digital ground planes); on the 3th, put power planes (analog below analog and digital below digital); on the 4th, signals again.
Third, separate digital and analog power thru a ferrite bead (check the resistance is as higher as possible at the frequency of your noise) which has capacitors at each pins. Capacitors could be 0.1u, 0.01u and 10uF large. Put also capacitors to bypass anyother noise you could have (GSM for example).
Forth, at each power pins, put suitable bypass capacitor for noise, in such a way to bypass noise to ground.
Fifth, put crystal really close to the device whom it is connected but distant from any other signal traces.
Hope to be helpful.
Let me know.
N.