What are good practices for traces on the "mains" side of a relay?
what you're interested in is creepage and clearance distances. Creepage basically is the separation distance of copper traces. Clearance on the other hand is the air gap separation of leads/conductive surfaces.

From the linked page:


  Typically, most standards are based on conditions being pollution degree 2 and overvoltage category II. It is important to note that as working voltage, pollution degree, overvoltage category, and altitude increase, the creepage and clearance distances also increase. The altitude is particularly important when testing to EN 61010.


Using the data from table IV, if we choose a working voltage of 125VAC, pollution degree 2, and material group IIIa or IIIb ther is a minimum of 1.5mm creepage required.

Clearance is calculated using the peak voltage, for 120VAC that's ~170Vdc. this table has the appropriate air-gap clearance requirements. Assuming peak voltage of 210Vdc, that means we will need at least a 2mm clearance.

Personally I like having a comfortable safety margin, so at the minimum I would probably go with 3mm creepage and 4mm clearance, with more being better.

There are a few other circuit protection devices you can add to your system to prevent spikes such as Metal Oxide Varistors (MOVs), Transient voltage suppression diodes, circuit breakers/fuses, etc.

Another route you can take is adding a physical barrier between devices, either by filling with an appropriately rated dielectric material (known as potting) or using a physical non-conductive shield.

This article has some other good high-voltage PCB design suggestions. Granted, they are generally catering for operations in KV's, but it does get you thinking about some of the potential problems and possible solutions when dealing with higher power PCB circuits.