[Plugin: Custom fields shortcode] very easily you can make custom shortcode

Put it on the function.php in your theme.

function caption_shortcode( $atts, $content = null ) {
return ‘<span class=”caption”>’ . $content . ‘</span>’;
}
add_shortcode( ‘caption’, ‘caption_shortcode’ );

Now it ready to use.
example: [caption]something content goes here[/caption]

http://wordpress.org/extend/plugins/custom-fields-shortcode/