Start off by making it clear that you're not like the other employees by creating a small difference between you and them. They have to wear a plastic hat, but you don't. Now, whenever they see you they'll know you're better than them. Not wearing a hat will literally prove to them that nothing is above you. You are the ruler of this place, and they are merely your peons. Don't be above reminding them of their hat wearing necessity every few minutes by scratching your head, waving your hair, or adjusting their hat while saying "here you go, now it's straight".
Next, enforce a more clear hierarchy system by making everyone refer to you as "manager." By calling you by your title and not your name, they will start respecting you more, until they completely forget the fact you were once one of them, or that you used to be lab partners. As time passes, they will feel less and less comfortable approaching you, until they will start coming to you only for their most important problems.
Enforce your power by denying your workers certain things they are already used to doing. If two employees come to you saying they want to trade their shifts -- don't let them. On the surface, it seems like this shouldn't be a problem since they are both willing to trade, and it doesn't really matter which one of them shows up for work and which one goes to a Lumineers concert. But this kind of arbitrary abuse of authority will show them who is really in charge!
Don't stop there, you can alter a lot more small things that are technically in your jurisdiction. Make everyone stay three more minutes every night to "recount the registers." Enforce a new dress code that makes everyone wear their name tag on the left instead of the right. If you really want to show your authority over your employees, only allow them to take one ketchup bag per meal. But don't forget, all changes must be small enough to go unnoticeable by your higher ups when they come to inspect you, and only serve as tiny little annoyances that will drive your crew insane.
This is what you signed up for when you got this job. This is where you show your true strength. There's been a theft from the register. Someone took a few cents, even a dollar, from their latest order. This is the time to show your employees your are not beyond torturing them to get a confession (in the context of your permissible rights, of course). Feel free to lock everyone in the store, and not let them out until someone confesses. Even if they say they will each put in 15 cents to balance out the register, don't let them!! This is the sort of thing that later leads to forming unions!
Walk around the store all cocky, waving your extra two bucks in everyone's faces. Buy a song on iTunes and play it repeatedly. Show everyone your clean shave using your new two-dollar razor blade. Pointedly give the extra dollars to the homeless guy in front of the store and talk about how you are a charitable person all throughout the day. If you really want to get under their skin, buy a lottery scratcher for a chance to make those 2 dollars into 200 thousand.
They used to hang the key on the hook on the wall. Not anymore! Now when your employees want to use the bathroom, they'll have to go through you. This is the ultimate power. This is what every manager dreams of. Controlling your subordinate's bladder is the last step to controlling their lives. You are now basically Bill Gates, Donald Trump and Rupert Murdoch all rolled into one. At this crucial junction in your career you'll have to decide what kind of manager you are. Are you the good boss who will give out the key right away, or the evil boss who will wait until just a little goes off in their pants?
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