OK, let's make a shabby polka dot background. There are two ways to achieve a worn, shabby look. The first is to use a combination of overlays on top of your background pattern. The second is to use erase brushes to erase some parts of your background pattern. I prefer overlays (it's much less work compared to erase brushes method).

Overlays:

The most commonly used overlays to create a shabby look are: old paper, crackled wall textures, sand/stone/concrete textures, old fabric/canvas. I often use sand/concrete textures.
Open the file with your background.

Add a texture (usual high-res photo) on top and change the layer mode to screen, soft light or overlay. Adjust the opacity. You might want to add several layers to achieve a desired look.

Eraser:

Open the file, click on the quick mask icon on the layer palette. Now you have to choose black as your foreground color. Select the quick mask layer on the layer palette to be able to edit it.

Choose a grunge looking brush, adjust its opacity and brush over the areas you want to erase to create a shabby look. If you don't like how it looks, just brush over with the white paint to restore it to its original state.

I suggest you try both methods to see the difference, and then post your results. ;)