Good question, Wolf. Here’s our take:
I try to keep up with the 10% goal over the course of the calendar year. For example, March (right before pesach) is when I give a fair ammt, but June-Aug tend to be a little softer. But the income keeps coming, so every now and then, I’ll run a subtotal to see how much I need to give to close the gap, and then think of who / how much to give so I can catch up.
Mrs. BP takes 10% right off her paycheck on the day she gets paid and keeps in in an envelope for the tzedoko auctions and parties she goes to over the year.
Small ammount that are given in shul during davening are not as systematic. If I have, I give. If not, I dont.
One thing I do try to do, is show my kids how much I’m giving and what my reasoning is so they can begin to delevope a sense of judgement of their own. I also show them that if in 2009, I gave xyz $100 and abc $50, but in 2010, abc has a much more pressing need, I’ll switch the alloaction. Its not an exact science, but at least it gets them thinking.