Ask HN: How to design a karma reward system

I am building what is actually a large portal for a 'tutoring site'. Visitors will be able to join groups, create groups and post comments, lessons etc. They will also be rewarded for completing on-line exercises.

I would like to introduce a 'karma reward system' in order to a) motivate people to contribute and b) to crowdsource ratings both for people as well as content.

I am leaning to something that is in-between what http//thesixtyone.com offers and HN's or stackoverflow's floating method of ratings.

I am looking for suggestions, formulae and good links to resources, books etc as well as other sites than the above three that you favour.