If your website looks something like this:

<div id="header"> ... </div>
<div id="mainContent"> ... </div>
<div id="footer"> ... </div>

and all your content is in the #mainContent div (with a fixed width), then you'd use a graphics program, such as Photoshop, to create an image of the same width as your column that has a graphic to the left and to the right (something that can easily repeat vertically). Once that image is created, you'd use some CSS on #mainContent to add it as a background:

#mainContent {
background: transparent url('/path/to/image.png') repeat-y 0 0;
}

~Sovereign