Wikipedia offers a great article describing the screen-printing process.
It’s important to remember that every color you want used in your artwork means another screen to create, set-up, and print through. The costs connected with these screens depend on the techniques used.
It’s always beneficial to print more items than less because of the set-up charges involved. If in doubt about the final quantity of screenprinted items you’ll need, it’s often more economical to order more than you think will be required.
Drop shadows, shading and anything that blends from light to dark will probably end up looking like a series of dots and should be avoided. (This does not apply to single-color halftone gradients).
The following special requirements will add to your screenprinting costs: