This image was helpful. Having done a fair amount of reading into which types of practices improve one's lifespan the most, I'd like to use my intuitions to draft a loose list of which sorts of interventions extend the average resident-of-a-develop-nation's lifespan the most, per unit of effort expended.

From most to least impact per unit of effort expended:

• Wash your hands frequently. Decreases risk of death from: influenza, septicemia, and other infections (~4% of yearly deaths in the US).
• Encourage your employer to buy a defibrillator, or, as a distant second, encourage your employer to allow your team to take a day off for CPR training. Decreases risk from: heart attack (~5% of yearly deaths in the US).
• Have a sober designated driver. Decreases risk from: vehicular accidents (~2% of yearly deaths in US).
• Make sustainable improvements to your diet and/or exercise regimen on the margins. Decreases risk from: cardiovascular disease, stroke, diabetes, and other causes (>40% of yearly deaths in the US).
• If applicable, quit smoking. This would be #1 on this list, except for the fact that it seems like a very hard thing for people to do. Decreases risk from: respiratory diseases, many cancers, cardiovascular diseases, and other causes (>50% of yearly deaths in the US).
• Make sure that your vaccination record is up to date. Some vaccines, such as those for HPV and yearly strains of influenza, are optional, yet highly beneficial. Decreases risk from: various infectious diseases (~5% of yearly deaths in the US).
• If applicable, quit recreational drug use. My intuition is that the benefit-to-effort ratio here presumably depends quite a lot on the drug. (~1.5% of yearly deaths in the US, not including deaths from problems indirectly caused by drug use, such as cardiovascular disease).

The actionable items listed above are the big ones, and I feel like there's a notable gap between those items list above this point, and those below this point, in terms of effectiveness per effort expended. Continuing on:

• Wear sunscreen when skiing, hiking, going to the beach, and so on. Reduces risk from: melanoma. Also reduces the risk of developing a carcinoma, which is non-lethal, but is ridiculously common (~0.4% of yearly deaths in the US).
• Be pickier about your sexual partners. Use condoms, and use them properly. Reduces risk from: viral hepatitis, HIV (~0.5-1% of yearly deaths in the US).
• Decrease alcohol consumption. Reduces risk from liver problems (~1% of yearly deaths in the US).
• If applicable, sell your motor bike, and acquire a safer means of transportation. Reduces risk from: vehicular accidents (~2% of yearly deaths in the US).
• Take a driver's safety course (this may make you eligible for insurance discounts, too). Reduces risk from: vehicular accidents (~2% of yearly deaths in the US).

I expect that there's another appreciable gap in the effort-to-value ratio between ideas listed above and below this point:

• Follow the advice on supplements given here. Reduces risk from: cardiovascular disease, and other causes (~31% of yearly deaths in the US).
• If applicable, move to a safer area. Reduces risk from: homicide (~0.6% of yearly deaths in the US).
• Brush and floss daily. I'm not quite sure how I feel about this one.
• Apply sunscreen every day. Reduces risk from: melanoma. Also reduces the risk of developing a carcinoma, which is non-lethal, but occurs in common (~0.4% of yearly deaths in the US).

Also, there are a few things that seem helpful, but don't fit into this list in an obvious way. Cryonics, paying attention to your body, and taking care of your existing health problems are good examples of this.