I would back Neo about certifications, they can help someone already in place in getting a promotion, the employer knows what skills he has, the certification shows he has all the required theoretical knowledge also... But very little help in this world where it is your practical skills that matters most.
wisecracker's thoughts are also true unfortunately, leaving you with very little choice and gamble:
As everything evolves so fast, you have no changes in being good at all things required, doesnt mean to not know, the approach, is what saved me a few times, know things just as if you were a analyst, OS? know what makes an OS, what challenges are behind when you talk implementation, that means be able to choose the right OS for the requirement ( best perfs-stability../cost...) etc, that means also know what is available on the market...
And now what makes you different and your gamble: like wisecracker/neo's advice: if still in eng/admin, choose a domain of the OS, a part of the Kernel, that you master, but really thoroughly, that shows you are a guru on the subject, most employers will believe with such mastery, you can adapt yourself to their requirements, meaning if you are not quite what they required, they believe you will learn, to match... at their expenses because you were chosen...
Once in place , beware, cruel world, stay up to date, because what I've seen, e.g. in banks, evolution is not necessarily in internal, or even more tricky, in internal with a new team, up to date with last tech etc... and you find yourself maintaining something that will be ported... and at the end, you are out of work, and out of date...
My, a bit more than 2 cents...