I use spreadshirt for my store:
http://sisterclaire.spreadshirt.com/I chose Spreadshirt for two main reasons:
a) They have the most t-shirt color variety of any on-demand printing company I can find. I looked at a lot of t-shirt printing review sites before settling on this one, although I'm still thinking about changing if I find something better. It suits my needs for now.
b) I need on-demand printing, because I have no room at my place for inventory, the nearest post office is quite far from me/I don't have a Belgian driver's license/it's just too big of an inconvenience to me to be mailing stuff when I could be drawing. Spreadshirt's prices are pretty decent, too. I don't make much off my shirts right now, but it's something I'm willing to sacrifice for the convenience, at least until I'm successful enough to be able to outsource my merch business and pay someone handle shipping things for me. (Who knows when that'll be, though! In the meantime, I still want to sell merch, even if I get peanuts.)
However, I *am* looking at a screen printing company right now called
PrintMojo, who takes a bulk order form you, and then ships out the merch for you from your online store. I already have an account, and I think it's likely I'll end up switching to them. They offer a LOT more freedom as far as colors (ink, not just tshirts) but also the location of the design (I have a lot of designs in mind that extend past the little printing area square most on-demand printing companies limit you to.)