Sports fandoms are fierce, but the rivalries are even fiercer. Odds are you can't name a single person more passionate than a Yankees fan watching his team duke it out with the Red Sox, and that's exactly what's led some clothing brand owners to success.

Starting a clothing brand – especially a sports clothing line – doesn't require a whole lot of capital. All you need is some passion, a good idea and the drive to follow through. The tenacity to withstand the odd cease and desist letter (or recover from your home team's gutting, ninth inning upset) doesn't hurt either.

Start With a Small Product Line and Then Expand

Chris Wrenn, the founder and owner of Boston-based sports clothing line Sully's, started his sports line purely with accessories. He began hawking anti-Yankees bumper stickers, patches and enamel pins outside of Fenway Park to capitalize on the intense rivalry between Boston and New York baseball fans. Eventually, he expanded to apparel and recently rolled out products in 14 Target store locations. The brand now has a 400 square-foot showroom two blocks from the stadium where it all started.

When it comes down to it, you don't need investors if you start small like Sully's. According to Marie Claire_,_ you can start your own clothing line for less than $50,000, but Wrenn said that all you need is $200 to create your first graphic T-shirt (which is part of the reason why the industry is rife with knockoffs). The fewer colors used on a screen-printed design, the cheaper the shirt will be to manufacture.

Keep Your Idea Simple and Strong

Many sports fans dream of having their own sports clothing line, but they get bogged down by a complicated product line and ever-shifting brand image. A brand's success hinges on a strong, unique image, especially because hundreds (if not thousands) of brands are already hawking licensed or knockoff team memorabilia. Eric Solomon, co-owner of the Texas-based screen-printing company Night Owls, said that if you try to wear too many hats – or rather, manufacture too many hats – your clothing brand may suffer. Not only is it more costly, but it confuses your customers.

"Complicated orders like complex designs and tons of different garments aren't really saving you any money and also give your customers too much of a choice," he said. "A million options isn't always a good thing. Pick one thing and do it really well."

Take the Opportunities for Which You Don't Plan

Every business needs a business plan, but it's impossible to plan for every single opportunity. The most successful clothing brands take the opportunities for which they didn't plan and shift their business model to account for the success.

For Wrenn, Sully's was initially a side gig. He started selling his products outside of Fenway Park to help fund his record label, Bridge Nine. At the time, he was focused on local releases, but his roommates were in a band that he felt had national potential. The anti-Yankees accessories helped him raise enough money for their recording and promotion, as well as the recording and promotion of 60 more releases. Today, Bridge Nine is still going strong with almost 300 releases, and Sully's grew legs of its own.

"The T-shirt business was initially just a way to get the seed money to build up the label, but after a few years, I realized that I had something that could stand on its own, so I incorporated Sully's as its own brand," he said. "Now, all of these years later, the record label and Sully's share an office, and I bounce between the two throughout the day."

Don't Use a Trademark if You Don't Have the License

If you're starting a clothing line that's focused on sports, you may feel tempted to use a logo or trademark from one of your favorite teams, but that's a recipe for disaster. According to Wrenn, who dealt with a handful of cease and desist letters early on, sports organizations don't care if you're pressing T-shirts in your bedroom. If you violate copyright and trademark laws, you're facing a potential lawsuit. Until you're in a position to become a licensee (which can cost thousands of dollars and major royalty percentages), it's best to create your own ideas and slogans.

"When we started selling to fans after Red Sox games, we were among the first to create our own intellectual property and protect it," said Wrenn. "I started putting the slogan 'Believe In Boston' on tees. Ben Affleck wore one in the movie 'The Town.' Red Sox players have been wearing the tees for years on the field and in interviews. It's become its own thing, and my brand owns it."

Even then, sometimes your ideas may walk a fine line, especially if you're dealing with teams and players. In that case, you may be forced to pull the product altogether.

"We had a T-shirt that parodied the 'Jesus Is My Homeboy' tee that was popular in the mid-2000s with an illustration of a player on it," said Wrenn. "It was shown close up during a playoff game on a fan in the crowd and we received a [cease and desist] from the player's agent, the player's association and the brand that made the original concept all in the same week. We just pulled the design and moved on."

Trademark Your Designs

T-shirt designs are easy to reproduce, which is why they're so susceptible to knockoffs. According to Wrenn, "Pretty much anyone with $200 can get a similar T-shirt out and commodify what you're doing." He said you should acquire a state trademark for your designs, which costs as little as $50 depending on the state. If a design is particularly popular, you should opt for a more expensive federal trademark, which can take a year or two and end up costing a couple thousand dollars.

"In [Sully's] case, that has been a worthy investment," he said. "Some of our best trademarks, like 'Don't Poke the Bear' and 'Believe in Boston,' are frequent targets for knockoffs, and having a federal registration number helps get them knocked down very quickly," he says.

Launch Your Sports Clothing Line

Once you decide on your product line and register your trademarks, it's time to launch a website. Many small business owners opt to use services like Shopify or Limited Run, which let you customize an online store without creating one from scratch. Others opt for online marketplaces like Etsy and Storenvy, which offer less customization but have a built-in user base. Neither is the right answer; it just depends on the business.

Get Creative With Marketing

Word of mouth is great, but a solid marketing plan is the foundation to a successful clothing brand. In today's climate, social media is a key tool to get your name out there, push your products and create a community around your business. The closer a customer feels to a company, the more likely he is to make a purchase. Just be sure to account for the ever-changing algorithms that can bury your posts or force you to shell out money in order to reach your followers.

"Instagram has been an essential tool in growing as a small business," says Ruby Cooke, co-owner of the accessories and clothing brand Life Club. "Learn how to use it to your advantage by understanding the algorithms. There are so many new, awful algorithms that are killing a lot of small businesses."

In addition to social media, some clothing brand owners like Wrenn opt for more traditional methods like newspaper ads. When Wrenn was alerted that Target released a T-shirt similar to one of his designs as part of their Local Pride line, he took out an ad in the Boston Herald and used the space to write an open letter to the company. The ad garnered him brand coverage in Business Insider, the Boston Globe and Boston magazine and ultimately resulted in Target pulling the T-shirt. Over a year later, the company reached out to Wrenn and asked to carry Sully's products in some of their stores. In July, they gave his brand a 400 square-foot showroom in one of their highest-grossing stores for sports apparel.

When it comes down to it, the most successful clothing lines learn how to make lemons out of lemonade (or Target showrooms out of near copyright infringement). At the end of the day, you just have to make sure you're always pushing forward.