can you explain your site more? I also have to say I like githubs pay for privacy or space.
A few of my current ideas are:
* Vacation days (a very standard feature... they extend the time limit on your games so you don't while you're away on vacation) (carrot)
* Advertising free (I don't have advertising now, so I'd have to introduce it for the free accounts) (carrot or stick?)
* 30-day free membership (stick -- don't like this idea)
* Limit on the number of simultaneous games a user can have (stick -- don't *really* like this idea, either)
* Prize tournaments for members only (need to research this idea... not sure I like it) (carrot)
And I'm considering a bunch of other minor perks (customize your profile, change colors, etc...) that would count as very small carrots.But more carrot than stick. If you hit your users with a stick too hard they won't want to pay and will leave, they may even talk to other users who wouldn't have cared and convince them to leave.
Perks are good. If you have any kind of social aspect consider having people advertised as paying. Theres a certain... prestige gained by letting people know that you are paying for a product that can be mostly used for free (at least I think there is).