Yet, these people are often not the most generous givers, because while the dollar amount is higher than others, they are giving a smaller percentage of their income or assets.
This system is a class-based system, giving more power to the members of the church who have the higher position in the class system of the general society. And like the general society, this class power is hidden and disguised. It means that our congregations conform to the class system, not counter it.
Tithing establishes a different standard: by explicitly creating an explicit standard based on commitment to the church, not on social class, it establishes a counter-cultural norm. That's how you fight classism.
Here is what I would recommend.
1. How to Start a Tithe System. Establish through discussion with the most committed members of the congregation a standard of tithing. Start with the 5 families who seem to be closest to a tithe right now. The ideal mechanism for transitioning to a tithe would be that 10% of the congregation makes a public statement that they have voluntarily agreed to meet a commonly held standard of tithing, and that they invite anyone else to come along.
I would avoid a general discussion. Why have people who do not intend to reach this level of generosity have influence over the discussion? I would certainly avoid a by-law change, which opens it up to a general vote. Why have cheap people have a vote on the level of generosity of generous people?
2. Defining the Tithe: There all kinds of way to define a tithe. You can have different standards for different life stages, a lower percentage of younger people. A percentage of net worth for retired people. You can have a path for growing the congregation into the tithe, saying "this year our standard is 2%, and in two years it will be 5% and in ten years it will be 10%." I would think that your starting definition would include some families already and be within reach of some more.
3. Verification: Once the standards are clear, the enforcement is on the honor-system. I would use the tax year as the standard unit of measure and make the designation of the tithe retroactive, and make the designation after April 15. (If I can say that my contribution in 2012 were x% of my 2012 income, I am on the list -- even if I didn't announce that intention at the beginning of 2012.)
I used to wish the UU World would annually list everyone who meets their churches' reasonable standards of tithing. But I rather keep it voluntary and self-directed: the churches with tithing systems should buy an ad in the World to announce their tithing members. Why get anyone else involved in the decision? Why make the UU World staff decide?
5. What about non-tithing members of the congregation? They should keep giving at levels which with they are comfortable. But as Chris Rock says, "what do you want? A cookie?"
I would like to eliminate pledge campaigns, pledge-based budgeting etc. They are also mechanisms for giving cheap people with low commitment to the church power to act as a brake on the congregation, but that is another post in what I guess will be a series: How to fight classism in UU congregations !