Today I thought I'd...well I don't want to say "switch gears." Let's say today I thought I'd veer to the left a bit. Instead of talking about designing in particular, I'm going to write a bit about being a designer, and how to get yourself into the business of t-shirt design. If you're an amateur custom shirt designer, or even someone who doesn't consider himself a designer, there are a few different ways that you can make some good money from your current stock of designs, and get some good exposure along the way.
There are three popular ways to try to make some money off of your work. The first, more obvious one, is to simply create your own site where you sell your work. Artists will use inexpensive custom shirt printing sites like ooshirts to purchase bulk orders of their shirts, then sell them on their own site. This is great because you will have control over absolutely everything, but this also means that you will need to generate your own traffic. People will not go to a website they have never heard of, and search engines will not put you very high on their list of search results.
Your next option is Threadless. Threadless has a cool system: anyone can submit designs every week. All of the designs are voted on, and the top five or so are sold for the week. Additionally, the lucky winners get a $2000 prize, plus a kickback from every one of their shirts sold. Some people have made all right money and gotten exposure from this process. However, winning is very difficult as there are hundreds, sometimes thousands of submissions every week. Shirt.woot has a similar voting-on-user-submitted-designs system.
Finally, you can set up your own mini store on popular sites such as zazzle and cafepress. These sites are incredibly popular and have lots of daily traffic, but you will run into the same issue you had when you tried to make your own site: you have to market your store. There are hundreds of unique, independent stores on each of these sites, so getting people to go to your page will still be your own daunting task.
T-shirt design is a fun, incredibly competitive business. It's best to approach it as a creative outlet and source of supplemental, rather than main, income.