Taking cyberjock's analogy a little further ...
Do you do simple preventative maintenance on your car, like: check the oil, other fluids and monitor tire pressure? If you get to the point where your brake disks are grinding, do you take care of it right away or do you wait until you have an accident?
Spend time reading the manual. Read it two or three times. Take a look at cyberjock's guide - it's in his signature. Peruse the forum for answers to questions you might have. If you can't find the answer - just ask.
After you set up FreeNAS, configure it to email you alerts about possible issues. Run SMART tests regularly on your hard disks, as well as scrubs. Keep a journal of things you've done and helpful commands.
If you sense a problem, check this forum first. Don't go to the shell and start entering ZFS commands that you found on a random website. Often people show up here, after the damage has been done. Or, with RAIDz1, they suspected a bad disk (was making noises), but put off replacing it for some reason.