Testing your vitamin D levels (either through your health care provider or with an at-home test) is an important step in reaching truly optimal 25(OH)D serum results (which is 50 ng/ml, for the record).
Once you have your baseline level, taking a premium D3 supplement with an efficacious dose (i.e., 5,000 IU) daily can help you achieve and maintain sufficiency (see a roundup of mbg’s top D3 supplement selections here). In doing so, you’ll significantly reduce your risk of premature death.