How about this one:
Or shirts like these:
Unless you've been living under a rock all your life, then you've undoubtedly spotted t-shirts similar to these. In fact, you might even own a few of these shirts. Custom clothing and popular culture have always had a close relationship. You can often get a pretty good idea of what a society is like by reading the slogans written across t-shirts and baseball caps. The emerging culture affects people, which ultimately influences their shopping and fashion preferences. If you're struggling to come up with design ideas for a line of custom clothing, then you can easily tap into the vast creative potential of popular culture. You can reference popular events, allude to famous people, and even steal slogan ideas. Image source: Downssyndromeassociation.wordpress.com
That last option has become pretty popular recently. Whenever somebody comes up with a truly catchy slogan that somehow manages to become part of popular culture, there will inevitably be dozens of lookalikes as people try to cash in on the slogan's popularity.
A pretty common example is the I [heart] NY shirt. You can find variants of that design all over the place: I [heart] LA, I [heart] TX, and I [heart] NJ. A more recent example is the Keep Calm and Carry On slogan. The catch phrase actually dates back to World War II, but it has recently undergone a revival as businesses and private groups adapt that phrase to suit new political messages.
Luckily, loose copyright laws allow companies to create spoofs of popular phrases. You can't really copyright a sentence, otherwise every sentence in the English language would already be copyrighted. Some individuals try to maintain control over trademarks and slogans, but at the end of the day it's nearly impossible to regulate a sentence or group of words. This allows you, me, or anyone else creative freedom to reference slogans like Keep Calm and Carry On. The other nice thing about these types of shirts is that nobody really considers it to be theft. It's almost as if the slogan has become part of the cultural identity, owned equally by everyone as people slap the slogan on shirts, bumper stickers, and funny Internet pictures. Consumers won't get on your case for reusing the idea because everybody else is already doing the same thing. If anything, stealing slogans will only make consumers even more excited about your brand.
Image source: Decorativethings.com
That doesn't mean that you have free reign to print whatever you want onto a shirt. You'd probably get into some legal trouble if you printed a shirt with company trademarks like, "Built Ford Tough." Phrases like YOLO, Haters Gonna Hate, or [Whatever] Rules are generic enough that you can steal it with worrying about getting angry letters from a team of lawyers.
Popular culture is a veritable gold mine for marketers. Don't be afraid to grab a metaphorical pick axe to cash in on the marketing potential. As long as you stick with popular phrases that aren't part of a company trademark, then you should be able to safely create trendy custom clothing.