What layout technique would you apply to reduce substrate noise coupling in a densely packed mixed-signal block?

We should  usually go with guard rings around the sensitive analog blocks. They really help to isolate the noisy digital parts. We place n-well or p+ guard rings and tie them to the right supply or ground. It’s simple, but it works well in reducing substrate noise, especially when everything is packed tight.

You can also reduce coupling by smart floor planning. We always separate analog and digital blocks as much as possible and try to route digital signals away from sensitive nodes. If you plan the layout well from the beginning, you won’t need too many extra tricks later. So I’d say focus on partitioning and placement first.