Hopefully someone will find this useful...
When you create or edit a template paste this code:
{set $properties['tpl'] = $_modx->getPlaceholder('template.tpl')}
{if empty($properties['tpl'])}
{set $properties['tpl'] = "default"}
{/if}
{include "file:templates/tpl/{$properties['tpl']}.tpl"}
Check 'Is Static' and set the media source to 'templates', usually you'd create one for snippets, chunks, templates etc. In this case our code above is looking in a folder called templates/tpl so our templates media source is pointing to a folder that has these folders within it.
Then in the properties tab click 'default properties locked' and add a new property set with the name of tpl and the value of 'base', for example. Then create your template 'base.tpl' in the '/templates/tpl/' folder.
Helpful Tips
Create an abstract template for your header/footer and inside your container div where you want content to sit within stick
{block "content"}{/block}
Then when you create a new template, for example a news template, you can extend the abstract template with
{extends "file:templates/abstract.tpl"}
{block "content"}
***your code goes here***
{/block}
Sometimes if you're creating a news template you'll have a different overview/containing page to that of the post page. Instead of creating two seperate tempaltes you can isolate the two markups with
{if $_modx->resource['isfolder']}
***Markup for your news container/overview***
{else}
***Markup for your news post***
{/if}
