I used 2 programs. Photoshop and Illustrator. Just browse the web for cool signs or images. Copy into PS and use Pen tool to outline. Save outline as a selection. Fill in with black, inverse selection, cut. New document, make it twice as big as default input from selection. Paste. Create new trasnparent layer and move below your pasted in object. Now you should have your black object with a transparent background. Next is how you want to make your sign. Diamonds are easy. Make a new selection on a new layer. For square just set at 512x512 and fill with white color. Make sure that layer is selected and rotate 45 degrees. Take that same selection and reduce it and fill with another color on another layer. That will give your diamond an outline. Then I save and open in Illustrator. Live Trace to convert to vector image, make larger. Save and open in PS again. Now you have a larger clean image. Next add text and spruce up however you like.
For getting the dirty grunge look, create a new layer each time you add a grunge brush, rotate them and move them around to your liking. I take base color and just change a tad bit. It doesn't matter if you spray outside the lines. You will take your base selection and inverse and copy merged. Paste into new document and size up. Done.
Those are very simple terms but it's a bit complicated if you have never done it. Read some tuts on the stuff I wrote if you get lost.