The rules of what to wear to work have changed - here's what they are now

Rule 2: Dress appropriately for your industry.

People who work in finance generally dress more conservatively than people who work in creative industries, di Giusto said.

You can't go wrong with a nice suit, but it might be a bit much for your workplace.

Rule 3: Know the purpose of your clothes.

People wear lots of activewear, like Lululemon's iconic yoga pants, to work — but di Giusto said this shouldn't be the standard.

"People misuse clothes when they go 'casual everything,'" di Giusto said, explaining that all clothes have a purpose.

Flip-flops, for example, are for the beach. Yoga pants, she adds, are for yoga.

"I promise you, 99% of those yoga pants never saw a yoga class," she said. "People tend to misuse clothes."

Rule 4: Don't wear hoodies and flip-flops. You aren't Mark Zuckerberg.

"As long as you didn't invent a billion-dollar company in your garage, you do not have the right to wear a hoodie at work," di Giusto said. "If you're not Mark Zuckerberg, you cannot do that."

The flip-flops-and-hoodie look, she says, is part of "Mark Zuckerberg's branding. That's his story."

But it's not everyone's.

Rule 5: Recognize that tech has made the workspace for casual, so layer and dress appropriately.

"As with fashion in general, workplace attire continues to trend more casual, driven by the start-up tech culture. Workplace attire is about versatility, outfits that can be worn in and outside of the office and from day to night. Layering and accessorizing is an important part of versatility, letting you tweak your workday look for a night event quickly," ModCloth's head of merchandising, Nicole Haase, told Business Insider.

Rule 6: It's OK to be relaxed, but don't be that guy. You know who he is.

"The combination of more relaxed dress codes and a greater awareness of the mental and physical health benefits of exercise have created workplaces where it's not uncommon to see people in activewear," Matt Taylor, founder and CEO of activewear company Tracksmith, said.

"That said, there's nothing worse than a guy walking around the office in brightly colored short-shorts. So if you are one of those people who brings your workout to work, be sure to dress accordingly," Taylor said.

Rule 7: You probably shouldn't wear a short suit.

Di Giusto can't wrap her head around last year's short-suit trend.

Rule 8: Put effort into getting ready in the morning.

One bygone trend is how much effort people used to put into getting dressed to go to work.

"It was like a ritual," di Giusto said.

Now, she says, "People just don't care anymore. They just wear a random piece very often and go to work."

She advises people to try to dress as well as they can — it represents their employers in a much better light.

Rule 9: Dress up your conservative clothing.

That's what retail startup Of Mercer set out to do — to let women who need to dress conservative still express themselves. The brand strives to create updated versions of traditional workwear — with flattering shapes and more flair than the pantsuit.

Rule 10: Embrace what makes you unique — it's not a pants-and-suit uniform world anymore.

We can credit high-powered women who defy fashion norms for these changes.

"Look at people like Marissa Mayer, Michelle Obama, who are saying, 'OK, I'm a powerful woman, I'm smart, I'm successful, but I'm wearing a bright-red A-line dress, and that's my power suit ... so really embracing what makes you unique and showing that off in the office," Dorie Golkin, cofounder of Of Mercer, told Business Insider.

Rule 11: Don't get too sexy.

"We still believe that a neckline that comes too low in the office is taking attention off of where it should be," Of Mercer cofounder Emelyn Northway told Business Insider. "And so I think that's one of the things that's sort of a hard-and-fast rule."

Rule 12: Balance it out.

"I think it’s really a balance of making sure — if you're uncomfortable wearing a really bright color, the shape of [your dress] is conservative," Northway told Business Insider. "Making sure if you are wearing a tank top, you're also not wearing a short skirt. There's sort of a combination of things to make sure that you're just not going too much in any one direction."

Rule 13: Streamline your wardrobe.

MM.LaFleur's Bento Box program takes its shoppers through a personalized survey to curate a "Bento box" of workwear. Each Bento Box is complete with four to six outfits of solid, professional workwear.

MM.LaFleur's team wrote a book called "Wear to Work: A Guide to Building Your Professional Uniform," that founder and CEO Sarah LaFleur told Business Insider "advises women on how to cultivate a streamlined wardrobe that will empower them at work."

Ultimately, a few great looks are better than a bunch of mediocre looks.