Perhaps it would be easier to figure out what is the worst ethical theory possible? I don’t recall ever seeing this question being asked, and it seems like it’d be easier to converge on.
Regardless of how negatively utilitarian someone is, almost everyone has an easier time intuiting the avoidance of suffering rather than the maximization of some positive principle, which ends up sounding ambiguous and somewhat non-urgent. I think suffering enters near mode easier than happiness does. It may be easier for humans to agree on what is the most anti-moral, badness-maximizing schema to adopt.
I’m not sure if framing it as a “sacrifice” may be the best phrasing here. Though it may be descriptively accurate to say that for most people who are giving, they mentally account it as sacrificial, we should try—where possible—to frame it as something positive and willingly done. This would probably make it more motivational.
http://effective-altruism.com/ea/4r/cheerfully/