Creating Grid Lines
Creating grid lines with photoshop is easy, and can be used to make your imagery look tight and froody.

1
Go into the channels window and create a new channel. Make a square selection about 20 pixels wide with the marquee tool. Fill it with white. Pressing the Alt and Backspace keys at the same time will automaticly fill the current selection with the foreground color.

2
IIn the menu, go:
SELECT > MODIFY > CONTRACT > Contract by: 1 pixel

Then, fill the selection with black. It will now look like a small white outlined box.

3
With the Marquee tool still selected, nudge the selection up and to the left, 1 pixel.

In the menu, go:
EDIT > Define Pattern

4
Clear the selection (Ctrl-D) and create a new channel.

Open the Paint Bucket options window and set the contents to PATTERN
Holding down Shift, make a circular selection filling up most of the image.

5
Fill the new channel by clicking with the paint bucket tool, and a grid texture will magically appear.

Drag the grid channel onto the "Load channel as selection" button. []

Go to the layer window. Create a new layer and fill the selection. (Alt-Backspace)

6
Enjoy the attractive imagery you and your new-found skills have just created and celebrate over a steaming bowl of re-digested monkey-brains.