This is what I call a 'COSMIC' question. Why? The business models are so varied and each may have a specific vertical or horizontal market they serve.
For example my focus is the extremely active running community. For me, the most successful marketing has been making sure my 'promotional' tee's are seen on the running trails. Yes, there are fewer eyes than other forms of marketing but those eyes have a incredibly high conversion rate.
A developing market is directly related to the proliferation of micro-breweries in the area. Again, my target market will be local and marketing will be via collaboration with brewery owners or promotion of the community 'vibe' and website.
For 'not local' marketing, I use 'long-tail' traffic drivers and blogs about the area and some sensational tee's they can't live without. These are ALWAYS accompanied with FREE T with purchase of THREE.
I do not rely on fulfillment providers but do collaborate with other local printers for the stuff I don't do. This has become a two-way street when they have small runs to print and press.
IF my business model was volume based, and my equipment could generate 500 garments a day, my strategy must be completely different. I would have two choices, institutional/company/team or online marketing to the general public. In regards to online marketing, nothing beats Facebook advertising, at this time. Hire a marketing company with developer access to Facebooks inner workings. This door is likely to close soon, but until then, reap the benefits revealed by Cambridge Analytics and get in front of those who NEED decorated apparel.
The options for marketing your business is limited only by your imagination. So, don't be blind to the obvious and use some signage. The bigger your ride, the bigger your sign should be. The message on the sign depends on who your ideal customer, as defined by the business plan.