I don't propose an exact way to do this. Perhaps, the way would be to simply stop deleting points for down votes. Deleting points for down votes gives an obvious disincentive for down voting, and down voting is what keeps the site spam-free and ensures high quality. Also, consider removing more points from the down voted question / answer. Currently the person down voting loses nearly the same as the person being down voted.
If a news anchor plugs a new product each day during his broadcast, he gives up a little bit of reputation as a trade off for boosting the other party's reputation. In SO a user has unlimited reputation to give away at no expense at no expense.