A simple (albeit limited) setup for caching static assets in WordPress. Sets far future http headers and then writes urls with timestamps. Full explanation at http://blog.poleteweb.com/2013/09/26/performance-tuning-wordpress-far-future-headers-and-cache-busting/

functions.php

function cache_buster_url($uri) {

	//Get base url of WP installation
	$base_url = get_bloginfo('url');

	//Get local path to the root of the WP installation
	$installation_root = $_SERVER['DOCUMENT_ROOT'];

	//Get path (from root) to file
	$path_info = str_replace($base_url, '', $uri);

	//If path has no dot (no file extension), return unchanged
	if(strpos($path_info, ".") === false) { return $path_info; }

	//Get full local path to file
	$local_path = $installation_root . $path_info;

	//Generate numeric timestamp based on modification date
	$cache_buster_date = date("YmdHis", filemtime( $local_path ));	

	//Add the timestamp to the file name, right before the file extension
	$url_chunks = explode(".", $path_info);
	array_splice($url_chunks, count($url_chunks) - 1, 0, $cache_buster_date);

	//Return full, rewritten uri
	return $base_url . join(".", $url_chunks);
}

sample.htaccess

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# Strip timestamp (cache buster) from filename
RewriteRule ^(.+)\.(\d+)\.(js|css|png|jpg|gif)$ $1.$3 [L] 

# Wordpress Pretty URLs
RewriteRule . /index.php [L]
</IfModule>

# Far Future Expires Header
# http://www.particletree.com/notebook/automatically-version-your-css-and-javascript-files/
# http://httpd.apache.org/docs/2.4/mod/mod_expires.html
<IfModule mod_expires.c>
ExpiresActive on
ExpiresByType application/javascript "access plus 6 months"
ExpiresByType image/jpg "access plus 6 months"
ExpiresByType image/jpeg "access plus 6 months"
ExpiresByType image/gif "access plus 6 months"
ExpiresByType image/png "access plus 6 months"
ExpiresByType text/css "access plus 6 months"
</IfModule>