A good way to get people to upgrade to s2members Pro is to include features that people want to use to customize their registration, profile editing, member management functions.
By default, Wordpress does not include a great user management system. Since you already have a foundation with which to work, you could work to improve the user management experience by offering features and functions that "fix" these deficiencies. While for the average user, they might seem trivial, to someone who is looking to create a membership site, pay or no-pay, it is a great boon.
You should look at custom registration fields, add a function to allow them to create as many custom user fields as they want and allow them to choose the type (single-line,multi-line,drop-down,select,radio,etc) and then you could allow them to edit the information that gets sent out via email for new members (welcome, welcome with password, lost password, unsubscribe, expiring subscription, etc) these options would make the Pro version very attractive if they have all these options to improve the normal membership experience.
These are just some idea, you could look at some of the question people have posted and decide to include those as Pro version features too, for future version of your script. Just an idea...