Make a realistic graffiti look with Paintshop Pro, by DuPz0r.
This tutorial can be used with PS too, but someone else can make that if they wish.
First, get a picture of a normal brick wall, a straight perspective would be easier.
Get yourself a font and/or a picture that you like.
Make sure the colors you have are bright so they constrast well with the brick wall.
Mess around with them untill you like the look of what you have.
paste your image over the brick wall on a new layer, and change the layer settings to Overlay
Duplicate this graffiti, and place it underneath the original one.
Goto Effect> Effect Browser> Presests> Adjust> Blur> Gaussian Blur> Radius 2 and apply it
And then put the opacity to about 35% (Adjust opactiy to how you think is best)
Your graffiti should now look like its been sprayed onto the wall realisticly.
When making your graffiti, make it light and a bright color, for the best results.
If you choose to do it on a white wall or something bright, then make the graffiti dull, and use Multiply instead of Overlay.