Preventive maintenance' procedures for WANs

Is there any 'preventive maintenance' procedures prescribed for wide area networks, which need to be carried out on periodic basis? (For a wide area network consisting of routers and the corresponding links.)
There are some very straightforward answers to your question. They would include sensible tasks that should be performed regularly such as

1. check for OS upgrades/security patches for routers
2. review interface settings of devices (I.E. via SNMP)
3. confirm fail-over mechanism functionality
4. measure end-to-end performance for
   1. comparison to SLA
   2. early indications of under-provisioning
   3. signs of poor management by ISP/WAN provider
   4. identification of hidden performance issues as well as others. I do not have a particular reference handy for you unfortunately.

Some form of network or device management system would be appropriate to support these tasks. Typically each system has its own framework that defines the various deployment and operational procedures. The framework itself requires regular maintenance to ensure it can accommodate your needs.

So unless you were planning on doing these tasks manually (OK for smaller WANs), the procedures that you can apply may well be defined by the choice of management system and the requirements of your network.

So what else is there to say? I would direct you to a related answer where I refer to Best Practices for Network Monitoring. Preventive maintenance must be balanced against its cost of course. And balanced against the cost to recover. Monitoring can be low-cost/low-effort and provide the next best thing to preventive measures.