I must admit the question is quite broad, so I'll try narrowing it down a bit. In our company we are 3-4 developers and have some SQL Server based installations running at our customer's sites (database sizes up to 100GB, up to 100 concurrent users, intranet applications). Nobody of us has real good experience in running/maintaining/administrating (whatever) databases. The customers not even that much. It's working ok till now, but I can't tell for sure if it's because we doing everything right or if we just didn't hit areas/situations we aren't proficient in.
So, I'm looking for the essential things you need to know when running a database from a DBA's point of view. You know the hard facts and know what matters most in your day for day job.
In which subjects should I gather deeper knowledge, what should I've heard of and what can I care of not until I face it for the first time?
I'm aware of the question Software Engineers and DBAs, but it's not quite what I was looking for. There are also lots of books around, but I'd like to hear it from those with practical experience.