You can download is a helpdesk webform process and some samples that demonstrate the Save as draft functionality.
To download the helpdesk webform process template, click here (C# sample only).
To download the web form samples, click here for the Samples pack. Samples 3 and 5 demonstrate the Save as draft functionality.
In summary, you need the following to implement the Save as draft functionality:
A webform button labelled as Save as draft
A process data to hold the value of the save as draft flag
A loop transition with a condition to check the process data (flag) value