Trapping for Screenprinting
By expanding the fill layers you allow your key or black layer to "trap" the color. This makes it a little easier on you if registration is slightly off. Even if you are not working with a traditional key, check your layers on the light table before you expose them so that you are sure no white paper is popping through and you have a little wiggle room when printing.
Trapping in Photoshop
To fill areas, use the magic wand and lasso tools to select an area on the key layer. Select any areas you can get away with- i.e. if there are lots of little black likes that will cover the color, just select that whole area.
Create a new layer
Select>Modify>Expand and expand 3 to 5 pixels so it overlaps the outlines
Use the Paint Bucket tool to click in the area and fill with black
In the layers window you can make the layer transparent to see how they interact
To save time with any repeated action try recording it in the Actions window and then press play to do it again to a selection. You can also expand all your selections at once to save time.