Mixed signal boards are not easy, since the digital noise will do it's very best to try and get into the analog signals (and it's often successful ;-).
So you want to keep the digital signal and signal returns as far from the analog circuit as possible.
Very important is to use separate analog and digital power and ground planes.
Assuming there are signals going between the analog and digital sides, the ground planes should be connected at only one spot (single-point) which should be where the analog and digital signals come together, such as a D/A or A/D converter.
There should be no other point where the analog and digital grounds are connected together (such as the power supply) unless there is only one supply for both the analog and digital circuits.
In that case it's likely preferable to run the power common to the digital side, but that may vary with the application.
You may have to try both ways to see which is best if you have a noise problem.
Connecting the two grounds together at the single point with a ferrite bead or surface mount chip can reduce the high-frequency ground digital noise getting into the analog ground.
Also decouple all the digital and analog IC's with 100nF ceramic caps directly from each IC power pin to ground pin, to minimize the noise generated on the supply and ground planes.
Surface mount caps connected close to the pins work best
If both sides must share a common supply voltage then a separate wire should go from the supply to the analog supply plane the digital supply plane.
In general, try to keep all digital signal lines as far away from sensitive analog lines as possible.
That's a few basic things that come to mind.
A search for
mixed-signal PCB design came up with a number of references that will likely have additional things to say.
Some suggestions may be different than mine, so you will have to decide which seems better.