This will depend on the climate at the location of the greenhouse, cost of fuel supplies, lighting requirements, temperature requirements, etc. Also you have restricted the answer to operational costs only so you can get some very capital intensive answers.
I perform a fair amount of energy audits on greenhouses. Greenhouses are designed to capture solar thermal energy directly. In the warm climate on the California Coast it is common for them to become too hot so ventilation controls are important. In cold areas anything that can be done to reduce night sky radiation of heat back to the atmosphere will reduce heating requirements. Photovoltaic solar energy would be useful only if the greenhouse had electric storage capability (batteries) or you could (electrically or thermally) heat water in an insulated tank and store that for cooler periods such as the nights or winter. Solar would only work if you had additional areas to locate the solar panels since you would not want to block the sunlight from the greenhouse itself.
A considerable amount of greenhouses have lighting fixtures which supply Photosynthetically Active Radiation (PAR) which provides the energy for the plant photosynthesis. When this occurs heating is usually not a problem since I have seen up to 680 watts of lighting for every 25 square feet of greenhouse area.
The most common forms of greenhouse heating that I see is gas fired (either direct infrared or convective through a boiler). Although this can be expensive the few number of days that is required in the main California greenhouse areas does not make the additional capital cost for more efficient systems cost effective.
I think the most cost effective measures would be those that reduce heat loss (or are able to control heat loss) through the greenhouse building envelope. I have not seen any of the earth tubes that tsgrue mentions but it sounds very interesting. I did see one greenhouse in Idaho that used geothermal hot water to heat the greenhouse.
