I heard a commercial on the radio the other day, pushing IT training as a way to become irreplaceable. That's the way to job security!
This is a bad way to get job security, for a lot of reasons.
First, let's take it from your point of view. Assume for a moment you achieve this goal. You're the one source for (for instance) information and solutions when it comes to your server systems. You're the go-to person. Then you get married, and you head off to a nice week (or two, if you're lucky) of honeymooning bliss with your sweetie. Your phone will ring in the airport while you're waiting to board your plane. Guaranteed. You'll have voicemail when you get off the plane. You'll spend a lot of time on the phone. Your significant other will not be amused.
I've been irreplaceable, and it sucked. My time was no longer my own. I had brought upon myself an obligation to serve the need I had created.
Now let's look at it from you employer's point of view. You have this IT guy (or gal), and he (or she) knows all. Irreplaceable.
Let's first assume you're a good person, and they trust you, and like you. And then you get married. And you're off on your honeymoon, and your phone doesn't work. Or you're having a child. You're in the hospital, and your phone is off. Something breaks, and you're not available. Sure, you'll be back in a couple of days, but now your employer is thinking, what if this person gets hit by a car? What if he dies? We're screwed.
Congratulations, you're now holding your company hostage, whether you want to or not.
Now let's assume that you're sort of a curmudgeon, prone to cynicism. Effective, but not exactly pleasant. At some point, your employer will think, what if this guy snaps? What small thing is going to push him from just cynical and unhappy to resentful and possibly vengeful?
Congratulations, you look like you could hold the company hostage at any time.
Either way, it's your very irreplaceability that might push your employer to find a way to replace you before accident or malice put the company's future in your hands.
So how do you achieve job security? I don't know. I'll tell you what I'm doing right now, that seems to be working well. I do the best I can. I try to be honest about what I can and can't do, and be honest with myself about what I'm good at and what I'm not good at. I strive consciously to maintain my drive and my focus, because that doesn't just happen, you have to push yourself to be motivated sometimes. You do your best.