Keeping your head when incidents occur can often be the difference between a speedy and successful recovery and a protracted outage period. To make sure you fall into the former category it is important to think about what you need to do in advance and document it, as it may not only be you who needs to follow it.
You need to define what an incident is with regards to your systems and services, perhaps have a number of categories defining the severity and actions to be taken.
You must keeps the lines of communications flowing between everyone who is affected without distracting those dealing with the incident to ensure that everyone knows who to contact.
Ensure an accurate record of the incident is maintained as this will be valuable when reviewing how it was dealt with, assessing the root cause and implementing improvements to mitigate it happening in the future.