in (some) order of importance:

- lots of sun WITHOUT sunscreen (diet and/or astaxanthin prevent sunburn), need cholesterol to biosynthesize vit. d

- reduce glucose spikes / increase insulin sensitivity with low-carb diet

- eliminate nutritional deficiencies: most effective is carnivore-ish diet in my opinion, meat and animal products are the best sources of bioavailable nutrients

- eliminate stressors in food: stimulants, coffee, etc.

- resistance / strength training

- cardiovascular training

- cold exposure (ice baths are awesome, I don't personally have access or a personal setup)