How to make grunge brushes:

This is just one way and a few tips on my behalf...

First find a good image - I like getting abstract style images from http://www.gettyone.com.

The following image is a section I cropped from such an image:

Now in photoshop go to: "image"=>"adjust"=>"desaturate" (this turns the image black and white - locate the equivalent in PSP):

Next go to "image"=>"adjust"=>"threshold" and this will result in the following - you can play with the threshold settings but usually I don't:

Now you can leave the image as is and already define the brush, but I usually go about the image and select bits that would make suitable brushes, or delete sections until what's left is good for defining a new brush. In this case I selected a certain area and filled it up with white and I left the rest to be defined as a brush:

Basically this gives you a lot of options and you can do this on larger images to get great results.

This is just a sample piece using the above defined brush. I rotated the image in all four directions and defined it four times, then I stamped over and over in black, white and colour... (You can even use this image as a brush...):

Other things I do - search for images that I then run the brightness and contrast on until what's left is great for defining - I usually select sections to define and don't define the whole image.

And of course, create some item, then start splashing it with other brushes, with white and black and with colour burn and colour dodge and also run filters, and repeat the effects.

Select links on grunge:
*Dubtastic's grunge border tutorial
*Dubtastic's grunge tutorial
*CarbonFour's grunge tutorial
*Wasted Youth's grunge tutorial
*NeoFrog's desplacement tutorial