Disagree with a set number of posts as it contributes to dribble. Post for post sake vs something of interest. My opinion is you either have it or you don't offer it as part of the whole system.

If you are going to charge then charge everyone, not just a select people to pay and another not pay. If the person is paying do they actually win something, if not why would they pay. If you can win something why should the person who is not paying any money have the same chance of wining something.

Using how long you are a member will not motivate people to post, all they will do is join and wait the period time then come back and enter.

I must mention that most of the time when I joined I did not post, I read the threads and knew what was going on but did not join in until I had an idea of what was what and who could take a comment and who could not. Then I started posting. If I Posted everything I thought my postings would be 4 or 5 times higher than what they are, but most of it would have been dribble or repeating what someone else has already said.

Realistically it comes down to what you want from the system;

1. People to post for discussion of cricket - I can not see any way to increase this.
2. To limit the amount of people in the fantasy games - set a quota system, x amount of places per game. Preference comes to first up first served at a set time to join the game (say a 2 day window to join) if you have more than the set number, then a preference system is set up - ie new people to the game have preference over ones who have had a game. Or any other system that the board feels as far.
3. Make money - charge everybody, people who pay you know are going to contribute.

Just my thoughts, waiting to be ripped apart