Well this one is not really a skill, but quite a useful guideline or precaution to follow - and consistently: Make your actions reversible (as far as possible)

Well this one is not really a skill, but quite a useful guideline or precaution to follow - and consistently: Make your actions reversible (as far as possible). Had learned it in the field, later also saw it mentioned in a sysadmin book. I can't count the number of times I've seen colleagues just blindly charge ahead and do, for example, non-trivial surgery (a.k.a. edits) on important system config files (in a haphazard, lets-see-if-this-works manner, without so much as making a backup copy of the file), which sometimes resulted in screwing up the system further, sometimes irrevocably (because, you guessed it, no backups - or no current ones, anyway). And yes, on production systems. Got a few stories about that, may mention them some other time.