Also make sure that you have accessible logs that log useful information (timestamps, the user making the request, unique request ID). Then use syslog or a SaaS service to aggregate logs from all servers in one place, and keep them for as long as you can.
To start with get something to monitor errors/exceptions and email you. To name a few services:
https://airbrake.io/
https://rollbar.com/
https://github.com/errbit/errbit (can be hosted on Heroku for free)
Also make sure that you have accessible logs that log useful information (timestamps, the user making the request, unique request ID). Then use syslog or a SaaS service to aggregate logs from all servers in one place, and keep them for as long as you can.