I found this list of causes of death by age and gender interesting (and if anyone has links to statistics that I can subdivide further, e.g. by race and socioeconomic class, I would be very grateful), although it doesn't immediately suggest what the lowest-hanging fruit as far as increased life expectancy are. I was most surprised by how high poisoning is on the list for my age group and gender (and others might be surprised by how high suicide is, or maybe not). I'm not sure exactly what this entails but my guess is it includes overdosing on stuff like painkillers, so don't do that. With increased age we get the usual suspects like heart disease, and I imagine these have been extensively studied.

Given a large enough database of deaths it seems like you could build a naive model that outputs a probability distribution over remaining years of life based on some key variables; I might be interested in doing this at some point if someone wants to point me to such a database.