Have you considered the idea of limiting self-votes to a few times per day?
The extreme idea is: No more than (IDK) 2-3 self-votes per day no matter what.
The less extreme: The vote power scales down with each new vote for yourself in a 24 hour period - first one can be 100%, next one restricted to 90%, next one to 80%, and so on down to 10% and then 0. Jerry Banfield had a great post on this.
Doesn't prevent collusion between multiple accounts though... and I guess now that I am typing this, I am realizing that it would be trivial to create a new account and upvote that one instead of your own. Hmm...