I would suggest giving points based on how long they've been a member by the number of days. For each day, perhaps give them 10 points or something. There are some excellent hacks that do what you want but from personal experience, it just made more people spam.
There's nothing worse than seeing a

as a reply. I modified it so people now get money to spend based on how long they've been a member of the site and posts give the user no reward, except for posting because they like to have a discussion, not raise their post count for points.
Something to think about anyhow.
Syl...