The Dunn article was published in Science, which means that most of the details are in the supplemental materials. Here's the relevant part:

Just before receiving their money, participants were asked to complete the Positive and Negative Affect Schedule (PANAS; S16), as well as reporting their happiness on the same single-item measure used in the previous studies. That evening, after spending their windfall, participants again completed the PANAS and a modified version of the single-item happiness measure (specifically, participants were asked to rate their overall happiness that day on a 5-point scale anchored with the words “not happy at all” to “extremely happy”). We standardized the 10 positive affect items of the PANAS and the single-item happiness measure to create reliable 11-item indices of happiness both pre-windfall (α = .81) and post-windfall (α = .87). A preliminary ANOVA on prewindfall happiness revealed no between-group differences, [F’s < 1], enabling the use of ANCOVA (with pre-windfall happiness as a covariate) in the main analyses of postwindfall happiness.

So happiness was measured with 11 items, 1 directly asking about happiness and 10 asking about positive emotions. Each item was rescaled so that the average of all subjects on that item was 0 and the standard deviation was 1. Then the 11 items were averaged together. Those who were instructed to give to others scored .37 points higher on that composite happiness measure at the end of the day than the spend on self group, controlling for scores on that composite happiness measure at the beginning of the day. Since the SD of that composite measure was about .64, that means that they were about .6 SD's happier, which is generally considered a "medium" effect size.

Similarly, random acts of kindness only make me happier while I do them, but not afterwards.

I'd count myself as non-neurotypical (and just one data point, of course) but... I agree that RAOK are short-lived - but that short-lived time is fun enough to keep me doing it every so often. I think it also helps to see that kind of thing as a sort of game. Making it fun makes me happy when i do it more frequently (though admittedly not very often).

As to giving to charities. I don't have a regular charity-donation because that is boring. I do, however, randomly give a years-worth of donation to charities that strike my fancy (Sea Shepherds, SIAI, Methuselah Foundation and more)

Perhaps non-optimal from their perspective, but it increases my happiness. Perhaps I'm aiming more at warm-fuzzies than utilons, but it works for me.

One of the other commentors speaks of how to combine this effectively - ie monthly setting aside the cash in "charity account" then being able to donate from this at will - which sounds like a good strategy for keeping it more fun, while still maintaining your pre-committed optimal give-rate.

As to giving in general. I realised a couple of years back that I was a bit of a tight-fist... the kind of person that never bought a round of drinks - and I have been actively working to change that behaviour pattern (eg by shouting lunch for my friends every so often, buying a plate of chips at a meetup etc). Even though I haven't changed very much yet, I have actually noticed a marked increase in happiness - albeit fleeting... there's a nice warm-fuzzy you get from spreading largesse.

...but the long-term effects are that I can now consider myself not to be so tight-fisted. My definition of myself is changing to one for which I have far more respect. That alone is worth the effort (for me).