Today's world is more competitive and cutthroat than anything we've experienced before. College graduates who have spent years and thousands upon thousands of dollars (much of that borrowed) can't find jobs and have to resort to fighting dirty to be hired for those jobs for which they've been trained. If you're going to be taken seriously in the business world, you need to take yourself seriously in the first place. Everything about you needs to show your employer and potential employers that you are ready for the responsibilities ahead. You need to show them that you're ready to be successful.
As much as we'd like to say that we judge people based on what's inside as opposed to what's on the outside, that's just not how the world works. Image matters, and while getting to know people and making judgments based on who they are is an important life skill to hone, the men and women who interview you and employ aren't going to take the time to genuinely "get to know you" before they make their hiring decision. Your image needs to say the right thing, convey what's inside of you to people who don't have time to discover that on their own.
Try and look at yourself from the perspective of others, people who don't know you, and try and define what you see. What do you think people first think upon meeting you? What does your image say about you? Does it say that you take yourself seriously as a worker and professional and that you're willing to go the extra mile, or does it say that you're kind of sloppy and that you'll only go as far as is required? While that's maybe not how you are—sloppy or whatever—your dress may convey that impression, and that's definitely an impression you don't want to pass on. You need to dress to impress; sure it's an act sometimes, but it's an act that's important to success, and it's not a superficial act entirely—rather it's a way of presenting yourself in the best possible way.
You may have heard the cliche "dress for the job you want, not the job you have." It's true. Sure you may have worked in a casual retail store to get yourself through college, but now that you're looking for a job in your chosen career field, you can't go around in jeans and Dockers. Dress for the job you want to have, and potential employers will notice. Invest in a few key pieces—a suit, some nice shirts or blouses, classy yet reserved shoes, professional skirts, and so on. It may cost initially, but it'll be worth it.
Once you get the job, don't slack off on your appearance. Rather, keep up your professional look to help show your employer that you really were worth choosing. Plus something else about dressing professionally is that if you dress professionally it has an impact on your attitude—you start acting how you look, and you're able to access parts of you and extend yourself in ways that you might not have been able to in jeans and a t-shirt.
Dressing for success is recognizing that appearances do matter and reacting appropriately. Dressing for success isn't a superficial game but is rather a way of being who you want to be and eventually becoming that. Take yourself seriously, show that in your dress, and employers will take notice.