Have you tried One-Time-Offers? You can find them under WP Admin -> s2Member -> General Options -> One-Time-Offers ( Upon Login ).
Basically you offer members some special something when they login certain amount of times.
Hope this helps.
Now officially accepting Professional s2Member installations along with Bruce C (a.k.a. Ace). If you're interested in a Professional s2Member Installation, or a Custom Coding Job, you can send your request here.
Or you can use a conditional in the Login Welcome page, checking if the user is at Level 0 (free) to offer him the upgrade, and if he has a higher level then not. WP Admin -> s2Member -> General Options -> Login Welcome Page WP Admin -> s2Member -> API / Scripting -> Simple/Shortcode Conditionals