What about having a vote budget? For example: say you have a collection of votes, each of which is one of -2, -1, 0, +1, or +2. Spending a vote uses it up, so when you've run out of (say) +2 votes you can't use any more of them until you get some more. Every time you vote, you gain back vote fragments in the following proportion:

1/9 of a -2 vote;
2/9 of a -1 vote;
3/9 of a 0 vote;
2/9 of a +1 vote; and
1/9 of a +2 vote.

This way, your total number of votes never changes, but over the long term you'll find yourself constrained to follow the curve. (The curve doesn't need to be bell shaped or even symmetrical, but this was just an example.)

The power of individual voters can be adjusted by doling out additional vote fragments to people according to some policy (much as I gather it works now).