According to the latest issue of the Chronicle of Philanthropy (July 2015) a new study by Russell James III, a researcher at Texas Tech University, will soon be released that offers some helpful insights into the sticky issue of bequest asks. The study showed that potential donors responded most positively to vignettes about other donors who had committed to a bequest and the benefits that will result from the gift in the future. The key point, italicized in my previous sentence, is that the vignette didn't need to be a story about a person who had already died, which would inevitably bring to mind the potential donor's own mortality, but rather could be about someone currently living who had made a connection with the charity.
Currently the study is a working paper titled "Encouraging Charitable Bequests: a Test Message of Effectiveness." If you have an interest in understanding bequests I'd keep an eye out for what sounds like an enlightening paper that could help us all with making a tricky ask less cringe-worthy.