HOW TO GET A SMOOTH GRID

Select the rectangular marquee. Hold down the shift key and drag-- it should form a perfect square. Remember, the smaller the square is, the finer the grid will be. Now, stroke the square (Edit/Stroke) however thick or thin you want your line to be. Now, expand your selection to about the middle of the line (Select/Modify/Expand)(the number of pixels should be half of the line width) and define it as a pattern(Edit/Define Pattern). Now, fill the image with the pattern (Edit/Fill/Pattern) and voila! There is your grid. To make the grid look like a "wireframe" computer model of an image that you like, distort the grid (Filter/Distort/Displace) (Note: this will only work if the image is saved in .psd format) and you should end up with an image similar to the one below.