1. Be rich.
2. Have an awesome significant other.
3. Cure for every current and future diseases.
Not to pick on you, but this is the reason I would be hesitant to use my wishes for something far-reaching even though you get the most bang for your buck - if you cure all current and future diseases, people will only die through murder/suicide/killing. Our world will get massively overpopulated and we'd be looking at laws like China's or looking at running out of water/space/food. I would be terrified that if I had a wish like "I want all my family and friends and myself to be happy" it would be at the expense of a stranger's happiness that I would never know.
1. Remove the human trait Greed.
2. Make writers stop ruining good TV shows/books/movies etc by adding awkward & unnecessary romantic subplots
3. Much more action against everything harming the environment
It seems you forget that dying of old age is not a disease. :P
Edit: Scratch that, I'd probably change it to curing cancer and stuff like that.
The definition of disease is "a condition that prevents the body or mind from working normally". No one dies of old age; people die because as they get older their organs get weaker and underlying diseases like heart disease are able to take advantage of their weakened state.