If the server is crashing there is nothing you can really do about that. If the server is going down for a restart and the admin is not posting an announcement well then just find a server where the people who run it are more considerate.
You can find some servers that have scheduled downtime and they will announce what those times are when you connect to the games or what not. Overall though there is not much you can do about an admin who is not considerate of the people in the game.. or the server just giving up the ghost... or zombie in this case.
On a secondary note, to avoid losing your stuff. Always disconnect when someone says they are taking a server down. It is the only way to ensure that your information syncs to the global data system behind DayZ properly.