Getting enough Vitamin D

The easiest and cheapest way for most people to obtain enough Vitamin D is by proving it is badly misnamed: get enough sunlight on your skin, and your body will make it for free. There are a few problems with this method, though not the one you're probably thinking of.
Problem number one: your age. As we get older, our bodies' capacity to make Vitamin D decreases. So much so, that it wouldn't be surprising for a 70 year old man with a deep tan to still be deficient. This doesn't happen overnight, and there's no sure way to tell if your body isn't "keeping up" without a blood test.
Problem number two: your skin color. Darker skin blocks the type of sunlight that triggers Vitamin D production. Even young people of African ancestry may not be making enough.
Problem number three: where you live. The farther you are from the equator, north or south, the weaker the winter sunlight shines. And if you live where it's often gloomy and overcast year-round, you're even worse off. Join the Polar Bear Club if you must, but don't think the wintertime skin exposure will keep your Vitamin D levels up - it won't.

So the only other practical choice is to swallow it. Question is, how much?
Again, the best way to find out is repeated blood tests, but this isn't the cheapest way. A good estimate, based on the best research available, is 4000 IU per day for an average adult.