I think from the age of 21 on, they should keep track of every individuals credits/debits to the federal govt. Once you are taking out more than you contributed, then if you are on any kind of federal assistance (welfare, medicare, food stamps, social security, unemployment, etc.) you must give up your right to vote if want to continue to receive assistance.
Included in your debit for each year would be your portion for defense, for courts, federal prisons, customs, etc.
Include in your credits would be interest calculated for excess payments that exceeded your debits in prior years.
Think about it: A welfare recipient who wants to keep getting a check has to give up their right to vote. A retired person who reaches a point where they are collecting more from social security than they ever contributed (including interest on payments they made) would have to give up their right to vote to keep collecting social security.
It would mean that politicians could no longer buy votes from those that are voting themselves money.
I think along similar lines - a vote should be weighted according to the taxes one pays. If you're paying more, then you stand to lose more by rogue government action and thus should have a stonger vote than a perpetual welfare bum.