What is the best way to heat a diy 8’x16’ greenhouse ?

http://homeguides.sfgate.com/heat-greenhouse-during-winter-water-21599.html

Passive solar heater? https://www.youtube.com/watch?v=pHk0PX8pbWY

Use manure in the 18th cent to heat there glass house for growing veg and pineapples they used manure pits. You could use a smaller version of this

There are heating cables out there that you can install in your greenhouse to keep it warm in the coming cold winter months

I knew a guy that used a small oil filled heater that turned on when the temp fell below a certain level.

The 1st thing you will want to do is figure out what size heater you need for your greenhouse. A greenhouse heater size calculator will help you with that. Then you can choose from their selection of greenhouse heaters or search for more options.