I made a comment on another site a week or two ago, and I just realized that the line of thought is one that LW would appreciate, so here's a somewhat expanded version.
There's a lot of discussion around here about how to best give to charities, and I'm all for this. Ensuring donations are used well is important, and organizations like GiveWell that figure out how to get the most bang for your buck are doing very good work. An old article on LW (that I found while searching to make sure I wasn't being redundant by posting this) makes the claim that the difference between a decent charity and an optimal one can be two orders of magnitude, and I believe that. But the problem with this is, effective altruism only helps if people are actually giving money.
People today don't tend to give very much to charity. They'll buy a chocolate bar for the school play or throw a few bucks in at work, but less than 2% of national income is donated even in the US, and the US is incredibly charitable by developed-world standards(the corresponding rate in Germany is about 0.1%, for example). And this isn't something that can be solved with math, because the general public doesn't speak math, it needs to be solved with social pressure.
The social pressure needs to be chosen well. Folks like Jeff Kaufman and Julia Wise giving a massive chunk of their income to charity are of course laudable, but 99%+ of people will regard the thought of doing so with disbelief and a bit of horror - it's simply not going to happen on a large scale, because people put themselves first, and don't think they could possibly part with so much of their income. We need to settle for a goal that is not only attainable by the majority of people, but that the majority of people know in their guts is something they could do if they wanted. Not everyone will follow through, but it should be set at a level that inspires guilt if they don't, not laughter.
Since we're trying to make it something people can live up to, it has to be proportional giving, not absolute - Bill Gates and Warren Buffett telling each other to donate everything over a billion is wonderful, but doesn't affect many other people. Conversely, telling people that everything over $50k should be donated will get the laugh reaction from ordinary-wealthy folks like doctors and accountants, who are the people we most want to tie into this system. Also, even if it was workable, it creates some terrible disincentives to working extra-hard, which is a bad way to structure a system - we want to maximize donations, not merely ask people to suffer for its own sake.
Also, the rule needs to be memorable - we can't give out The Income Tax Act 2: Electric Boogaloo as our charitable donation manual, because people won't read it, won't remember it, and certainly won't pressure anyone else into following it. Ideally it should be extremely simple. And it'd be an added bonus if the amount chosen didn't seem arbitrary, if there was already a pre-existing belief that the number is generally appropriate for what part of your income should be given away.
There's only one system that meets all these criteria - the tithe. Give away 10% of your income to worthy causes(not generally religion, though the religious folk of the world can certainly do so), keep 90% for yourself. It's practical, it's simple, it's guilt-able, it scales to income, it preserves incentives to work hard and thereby increase the total base of donations, and it's got a millennia-long tradition(which means both that it's proven to work and that people will believe it's a reasonable thing to expect).
Encouraging people to give more than that, or to give better than the default, are both worthwhile, but just like saving for retirement, the first thing to do is put enough money in, and only *then* worry about marginal changes in effectiveness. After all, putting Germany on the tithe rule is just as much of an improvement to charitable effectiveness as going from a decent charity to an excellent one, and it scales in a completely different way, so they can be worked on in parallel.
This is a rule that I try to follow myself, and sometimes encourage others to do while I'm wearing my financial-advisor hat. (And speaking with that hat: If you're a person who will actually follow through on this, avoid chipping in a few dollars here and there when people ask, and save up for bigger donations. That way you get tax receipts, which lower your effective cost of donation, as well as letting you pick better charities).