I’ve been having a lot of noise problems lately, both with boards and with equipment doing testing in the lab.  As a result, I spent a bit of time looking around at some solutions to help reduce the noise on existing boards (basically filtering since the noisy board itself is out of my control), and came across a good app-note from Freescale on noise reduction techniques.  A lot of this is focused on PCB design, and much of the information is available from other sources, but if you’re serious about designing good PCBs, it’s worth a read and is more accessible than some other documents out there on the subject.  A lot of noise reduction comes down to good grounding, and unfortunately it’s not something that’s always done half as well as it could be (and I’ve been as guilty of this as anyone, though I’m making a conscious effort to consistently improve).  High frequency return paths need to be kept in mind while routing and laying out your PCBs — not after the fact — since you can’t do much about it when you find out you have noise problems and poor performance once the PCBs come back and get assembled.  See AN1705: Noise Reduction Techniques for Microcontroller-Based Systems for some good tips and suggestions.