Concatenate $base_url and path_to_theme()
In hook_preprocess($vars) create a new variable called path_to_theme in $vars by including the following (where $hook is page, node, block etc)

function mymodule_preprocess(&amp;$vars, $hook) {
  $vars['path_to_theme'] = base_path().path_to_theme();
}


Now in virtually all of your theme's template you can use the variable $path_to_theme and it will be the contents of base_path().path_to_theme()

Note, that you can put this in a custom module, or replace mymodule with the name of your theme and you can place it in your theme's template.php

You can use node, page, and other "hook" specific preprocess functions if you only wish to add this variable to specific content of your site.

There are many options for preprocessing nodes, pages etc. which can be read up in:
Setting up variables for use in a template (preprocess and process functions)

Also be sure to read up on the Default baseline variables such as:


  $directory The theme path relative to
  the base install. example:
  "sites/all/themes/myTheme"