This is pretty much it. Take up a complex hobby that forces you to think and concentrate.
If you walk into the bathroom, catch a glimpse of yourself in the mirror, and don't recognize yourself for an instant, you could be in trouble.
Eat healthy (and natural), get regular exercise, and get plenty of sleep. This helps minimize the various stresses which are awful for the brain.
Don't worry too much about doomsday as this also causes a lot of stress. As I get older, I see doomsday more as a release and am disappointed when it doesn't happen. The AI Apocalypse will likely happen before you hit 80. When the nukes are falling, find a nearby major city, go to the top of a skyscraper, and have a viewing party.
When my mother was going downhill from dementia, she could have handled it a lot better. She knew she was having problems but didn't even try. Most people with dementia will reach this point, but if you program yourself early to double check yourself and follow lists, you'll do far better and be less of a burden on the people around you.
Be good to your kids... they will be the ones caring for you in your last days. One day they just might say: "Hey mom, why don't you go for a swim with Buddy?"