"jeazy" is nearly right, but forgot to tell you. If your image is in colour, go to IMAGE/MODE/GRAYSCALE this will turn it into a grayscale, then back to
IMAGE/MODE/RGB Then IMAGE/ADJUSTMENTS/PHOTO FILTER ten select FILTER and turn up to 100% If your image is a black & White, convert it to RGB then go from there. Philip.
Whilst Photoshop's sepia is OK I tend to use the actions from http://www.1clickactions.com. They have several sepia versions as well as other toning and split toning effects.
Pretty versatile and easy to adjust.
There are lots of ways. One easy way, that is customizable, and reversable, it to use a hue/saturation adjustment layer. Click "colorize", and set the hue to between 30-50 (usually 40 is nice), and the saturation from 15-30 (I like to use 17-20).
You can always change the settings by double clicking on the layer. You can also fade the opacity, change the blending mode, and add a layer mask to mask off parts selectively.
I like using duotones and tritones. Convert to grayscale mode, then select duotone. You can load the dutone, tritone, and quadtones from Adobe PS, or make your own. When finished convert back to RGB.