Not really, just know in your mind what you want the template to look like before you start out, what angles of the shirt, short, socks you want to show. Also make it on a reasonably large canvas so you can maximise the amount of detail you add, mine are usually made around 2000px then layers are flattened and resized down to about 1000px before uploading.
A good way of starting out may be to trace over and actual shirt image you find online then stylize it by evening up and rounding off.
Ok so their are a few tips
If I get time I'll try and do a quick tutorial on how to use the pen tool and the direct selection tool.
