So, when a design pops into your head, what steps do you take?
Good question!
Depends on the site (I have a few different t-shirt lines).
If it's an established line with a steady flow of customers, I will start out with a run of say 25-50 screen printed shirts, depending on how well I think the design will sell (ie. If I've gotten a lot of emails asking for a similar design I would order more...if it's my own twisted creation, I would order less).
I usually order like 5 Mediums 10 Large, 10 XLarge, 5 XXL (or some similar combination ratio).
I send out an email to my opt in newsletter base and post the design on the site in a few places and judge the initial sales of the shirt (I give it 1-2 months).
If the design does well, I reorder, if it doesn't, I don't.
For the screen printed designs that I fulfill myself, I know that they *will* sell because I've been selling the shirts for years. So having inventory doesn't bother me because I know I'll have the stock in hand when the orders come in and I know the orders will come in

Some designs I carry a LOT of stock for and some designs I carry minimum levels.
Slower times of the year I let stock levels stay low, but when busy shopping seasons come (spring/summer, holidays), I keep the stock levels higher.
If I have a new t-shirt idea that it *outside* my normal "established" t-shirt lines, I'll create the artwork myself (or hire someone to create it through designoutpost or one of the other freelance companies I use) and put the design up for sale in one my my cafepress stores. If it sells well, I might create a printmojo store to get the product screen printed or I might just leave it for sale at cafepress.
Depending on the market, if I wanted to start a brand new t-shirt line, I would probably go through printmojo, spreadshirt or cafepress .Each service has its benefits, but the main benefit for me is NOT doing the fulfillment myself, because when you have a lot of sales, it can be very time consuming to go through the orders, screen for fraudulent orders, print out shipping labels, pull the stock, package the shirts, schedule a pickup. If I had to also print the shirts at the same time, I'd probably go insane
