Tips for securing a server?

Somebody broke into my server (one of my clients who has shell access probably wasn't very careful with his username and password) and installed an IRC bot inside and compromised my server.

Everything has been fixed now but it kept on going down for no reason and the server has to be rebooted to be brought back up.

I dont' know what cause of that was and not sure how to find out.

The datacenter staff (thePlanet) aren't successful finding out either and their security team costs 75 dollars per hour.

Any recommendations?

 

 

 

 

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