Download my favorite grunge pack:
Brushes
2:
Start a new 450x150 transparent document.

3.
Load the brushes and select the second brush in the grunge3 list.

4:
Now do Filter>>>Render>>>Clouds.

5:
Now heres the artistic part. With this brush selected.... simply switch from black and white randomly(hit x on keyboard) and click. Until you get something similar to:

6:
Now that the artistic part is done, its all procedure.
First... Select a render you want to use. Then simply drag and drop to the grunge background you just made.

7:
Make sure u get rid of any rendering color(the white around the character)
I recommend the Magic Wand Tool.

8:
Now that the render is there press CTRL+SHIFT+U, to make it grey-scale (all grey).

9:
Now for some color. Go to Layers>>>New Adjustment Layers>>>Color Balance.

10:
Just mess with the bars until you get a color you like.
11:
Now click on your render layer, then click add vector mask.

12:Now get a grunge brush and make sure your foreground color is black.
Start randomly and SLOWLY click around the edges of the character until he is blended in like so.

13:
Now your pretty much done. Except, I always add a 3PX Border and some effects or other brushes over the background.