Tithing started because Abraham gave 1/10th of the spoils of a battle to the high priest Melchizedek in thankfulness to God for his victory. What's interesting to me is it's not clear he ever tithed before or again, and that he had clearly been blessed and was very wealthy, yet he only donated from the spoils of the battle.
My parents did a strict 10% tithe. My mom was bitter about it when i was young (actually still is) because things were very tight. She said "It felt a bit like paying off the mafia" which I think means she was a bit annoyed at a particular church for pressuring them, my father for doing it, and didn't feel like God was asking for that kind of a sacrifice. Now, currently they have a great 'stache, so giving 10% is very easy for them, but I don't think that they ever exceed that amount because they are very conscious of the fact that they must live off whatever they have. At the end of the year, my mom adds things up, writes a lot of checks to make sure they've met 10%. She's not bitter about doing it now, because things are good, but she certainly isn't going to "give until it hurts." What would that mean? Jeopardizing their retirement?
I'm in a church where there's no rule, and most people just toss some change in the basket. I think that annoys the priests a little bit. In fact, one once said, "Catholics are tightwads. People will drop $4/day on a latte', and then they put a couple of dollars in the basket." I got a laugh out of that, because sometimes he's right. (He was at the time trying to fundraise for repairing an aging Church and having trouble getting enough to just keep the bricks from falling.) But what it's done is pushed some people a bit the other way--I know a deacon who says "You should give until it hurts." I just don't agree with that either. I doubt Abraham was the least bit put out by donating 10% of his winnings. In the middle is the idea of stewardship--that you support your church (and charities) with a well-thought out portion of your earnings. They'll make suggestions but it's generally been around 3% of people's earnings.
I've decided to take the stewardship approach of "somewhere between 5-10% if I can do that." My husband is trying to save for a house, so I'm trying to be sensitive to his hard work by not "giving the house away." I don't advocate the "making it hurt" approach, but making it meaningful to you. "God loves a cheerful giver," and on the flip side, having worked at a non-profit, I think it's important to make larger donations to a smaller numbers of charities, because spreading it out $5 here, $10 there doesn't do a lot of good because there's a cost to having someone on your mailing list and a cost to the manpower of recording, receipting, and depositing donations. There's a benefit to having regular "large" donors who you can count on when you make a yearly budget. I think if you're really excited about a cause, you'll be happy to give generously. I think "making it hurt," takes the excitement out of that. Sure we needed money, but we didn't want bitter or grumpy donors.
The other thing you can do is, of course, to write a 10% or larger donation into your will (as smilla said). Thus anything you didn't spend in retirement and end-of-life care can be given as a tithe. I kind of like that approach as a good "Paying off the mafia approach" "You want the money, God? Quick and easy death please." :)