The day you vote on an idea, that vote is worth +1 or -1. After a week, your vote contributes +/-0.9 to the total, after a month that falls to 0.7, after three 0.5, after a year 0.3. If you vote again on the same idea or change your vote, the vote is worth +1 or -1 again. Each vote declines separately
without regard to how often you vote on other ideas.
This has the following advantages:
(1) new ideas get a score closer to that of ideas which have been around longer.
(2) votes made on a whim a year ago have less status than votes which are reinforced regularly.
(3) voting on old ideas is spread out over time rather than being concentrated on the first few days of their creation: this will allow for more balanced reflection.
(4) frequent visitors to the site have more effect than people who have given up and ran away.
(5) truly classic ideas will still be able to accumulate high scores.
(6) unlike with other vote normalisation techniques, this recognises that you may vote 'for' more often than 'against', or vice versa.
(7) the site will become a more dynamic entity, changing with time and increasing interest for visitors.
(I have not seen this scheme implemented anywhere, but would be interested if anyone has baked it.)