If servers are housed in a standardized and specialized server room then maintenance should be incredibly easy. The risk of heat and dust damage is drastically reduced, but the servers should still be given a good dusting, inside and out, every couple of months. It is important to note that servers do not receive the frequent reboots that a normal PC does. In fact, a server can sustain an uptime of well over a year. Typically, the server going offline leads to downtime throughout the entire network, so it is done as little as possible.
Mostly, server maintenance simply involves checking to see that every component is running as it should. Performance should be monitored daily on things like HDD Seek Time, Memory Errors, and CPU Capacity, among others. Fans should also be checked regularly to ensure they are still functioning.