how to prevent server crashes?

Install some prop spam protection and a good lag detector. If after doing that still crashing, remove some useless and heavy addons. In the pass i have the same problem with my DarkRP server, and it was an addon that was crashing the server, because it removed the speed limit from the props

I would set up an automatic restart on server crash, and you can also find many addons which may help to further prevent crashes.

This addon works fairly well: https://github.com/SnowboiTheGr8/gmod-anticrash
You can add -conlog to your server start parameter and check the console.log file for more details as well

use a anti propgriefing addon and make a auto server restarter when crashed