Include enough comments in your scripts that several years from now, you or someone else can easily figure out what even your most complicated commands are supposed to be doing. A few comments can save you a lot of time in the long run.
Patch your systems periodically. If you can. configure your software to automatically install security patches. Attackers aware of recently discovered exploits will be trying to get to your systems before you patch away their access. So don't be slow to respond if you can help it.
An old boss of mine used to describe the process of spending so much time keeping track of what you're supposed to be doing that you get little else done as "thrashing". Simplify the process of keeping track of your tasks so that you can more easily focus on one thing at a time. You might be surprised by how much time you save.
Stop doing so much busy work. Studies (such as one published in the Harvard Business Review in September 2013 called "Make Time for the Work that Matters") have shown that knowledge workers, including people like us, spend on average 41% of our time doing things that aren't gratifying and could be delegated to someone else. Spend more of your time planning what you intend to accomplish and setting priorities.
If you have the option, delegate tasks that someone else (maybe someone less skilled) can do and focus more on those tasks that truly require your expertise.
Periodically verify your backups as well as your disaster recovery plans. Don't wait until a disaster strikes to see if you're really ready to react. If you do, you might find out that, in an emergency, you're not as ready as you expected you might be. This readiness requirement also applies to preparing for a security breach. You might have a pile of plans, but if they're not tested and you're not practiced, it could take a long time just to figure out what you're supposed to be doing, who you're supposed to notify, what you evidence need to preserve, etc.
Keep acquiring new skills. After decades of being a devoted Unix user, I'm still learning new tricks and new tools. If you're not sure what to study, browse some job announcements for positions that sound interesting to you. You're likely to find some nice lists of required skills attached. They might point you toward some areas of focus. Always be learning something new. It's not just good for your continued job prospects; it's good for you and can be quite motivating.
Be versatile. Learn more than one distribution. There are some major differences, not just in the desktop, but in how the systems boot. Make sure you can move from one Unix system to another without a major setback.
Make time for exercise -- sitting at a keyboard all day is bad for you. Add a significant commute and it's really bad for you. Get up and walk around now and then. It will actually help your brain work better and enable you to get more work done.
flickr / robstephaustralia