Select the best tshirt printer for your needs. If you've taken the time and energy to create some great tshirt designs then by equal measure take the time to do some research on printers that will produce the best results for you, in a suitable timescale and at a reasonable cost. This advice sounds obvious but some would find it too easy to plump for the first printer that offers a cheap deal to see their tshirts quickly only to find the results are not as expected.
Gather a list of printers who work with tshirts. You can find plenty online these days so check out their websites, then fire off some emails asking questions about quoting a price, timescales, printing methods used, delivery costs. Do they supply tees and labels (a lot do) and if so what's the quality of those items? Can you supply your own tees/labels if it works out cheaper for you? I'm sure you can think of other questions and if so then ask them. Those that don't respond obviously don't want the work or are obviously going to be hard to communicate with. Out of those that respond look for a timely reply and answers to most if not all your questions.
Alternatively take a bold approach and try and find some recommendations for printers from current tshirt designers. You'll find most online folk are friendly and communicative so source some independent designers and ask who they use.
Consider using the printer's in-house designers. Just because you've done a great design for your tshirts doesn't necessarily mean it will print well. Most printers have their own designers who can offer advice or adapt your designs to make them print better. It will add to your costs but could be a huge saving in the long run if your tshirts come out at the best quality and sell well.
Use vector drawing software for your designs. It's better to use software like Adobe Illustrator or CorelDraw to produce your tshirt designs as you'll be guaranteed sharp lines in your artwork as opposed to the pixellated attempts in other paint programs.
Keep your designs simple. Basically the more colours you use in your artwork then the more complicated becomes the printing process. So limit your colours and keep the costs down, it may also help you focus and tighten your designs.