A good greenhouse has some of the best accessories and if you are building your own don't forget to have fans and vents to control the humidity and heat buildup in the building. One other way to augment the efficiency of the greenhouse is to use a covering to prevent the strongest of the sun's rays from entering during the hottest months.
A good thermometer can be used in conjunction with the fans to help adjust for the perfect temperature for the plants no matter what season of growth they are in. If a greenhouse is too hot it will cook whatever is inside, plants, and people both. You can setup greenhouse automation systems or climate control systems inc for efficient growth.
In the coolest winter months, you may have to add some method of heating to keep the temperature from freezing. There are several options for this, including heaters or even using solar collectors if you have the right exposure and sunny days to do this.
If you do use heaters, make certain that the greenhouse is insulated and the power outlets you're using are the right size so you are not wasting energy. Consider the square footage of your greenhouse and that will help to determine the size of the heater your need to have.
One thing to consider when constructing your greenhouse is the possibility of it being subject to high winds. The best way to prevent it from blowing away in the first squall that hits your area, if you live in a predominantly windy area add more tie-downs to secure the building.
Greenhouses need a good system for handling the plants as well as room to move easily when moving a larger pot or other material.