Have you ever tried not to read a t-shirt? People of all cultures, nationalities, and ages wear t shirts. As popular as jeans, t shirts can be wearable art, a means of identifying with a group (such as family reunions), a forum to express political statements, religious beliefs, or preferences running from wild life to wild parties, and everything in between. The key to the popularity of t shirts lies in its ability to be whatever you want.
Using t-shirts to promote your organization, company, or business is a wise choice. Just think about how many people see and read t shirts everyday. Having people advertise for you can cost you about the same price as a brochure; except the brochure may or may not actually ever be read. And even if it is read, chances are high it will only be read by one or two people. That is definitely not the case with a t-shirt! T-shirts are read multiple times a day by multiple people. "That's true" you may say, "but how do I get people to actually wear my ad?" It doesn't have to be an ad and you are more likely to peak a person's curiosity if it is not an ad. That's the key actually; to peak curiosity. When you can peak a person's curiosity, they will do their own research. And when they do their own research, they are more willing to buy into it.
Once upon a time printing t shirts to promote businesses was 'kind of neat idea'. Now it is rare to have a business identity program without a t-shirt. Businesses and organizations that have nothing to do with the fashion industry often have t-shirts produced for corporate identity, as PR material for sales promotions and special events. Often these t-shirts are worn by staffers as uniforms at such events, distributed as souvenirs or may end-up being distributed commercially. Brand awareness is still a popular trend and even as clothing manufacturers have turned their products into promotional vehicles in and of themselves, t shirts can take that concept one step farther by being clothing that promotes other products and services.
T shirts are indeed items of clothing - and inexpensive ones at that. This is obviously why the popularity of t shirts as promotional items is what it is today. You will be hard-pressed to find anyone who will simply throw away a new piece of clothing and most people will, at the very least, give a tshirt to someone else if they themselves don't want it. However, fashion is probably a more powerful factor in this than frugality. Take the classic tuxedo t-shirt as an example; most anyone can think a fun way to use this design and customizing it with your promotional text on the back puts it to work for you instantly.
This is the essence of fashion - people seem willing to wear someone else's message because they feel it says something about themselves. When products or services are promoted in a way in which the wearer is willing to be identified, t-shirts become a ubiquitous promotional vehicle. Simply walk down any urban street in the world to see for yourself. Let's once again compare it to a brochure or flyer: if you were to mail promotional brochures to 100 potential clients perhaps 200 people will see it. But mail out 100 decent looking t shirts instead and you launch 100 walking billboards!