Which energy efficiency measures make the most difference in reducing restaurant energy costs?

Something that is tied closely to your question is what is going to provide the best ROI? Replacing your HVAC system might technically make the most difference in reducing energy costs but be incredibly cost prohibitive until you have the capital to make the upgrade. For an already established restaurant I personally would look at tackling energy efficiency in a "debt snowball" like way, putting the highest emphasis on those things that can be fixed quickly and relatively inexpensively such as replacing traditional lighting with highly efficient alternatives. For a small outlay you can immediately start to enjoy savings. On a practical note, this is a very visible change that your customers can see and will notice, which will foster goodwill between you and your guests. Secondly I would look at the replacement plan for equipment and buy energy efficient upgrades according to your replacement pattern. Spending a lot of money on a two year old piece of equipment just because it would cut energy costs a little more than replacing the six year old piece of equipment sitting next to it, seems to be counterproductive. What is going to break first, replace first and spend the extra money then to get as energy efficient of a model as you can. By frying the small fish first you realize savings which frees up money for bigger upgrades, and by replacing equipment in strategic way you don't feel the pinch as much as your energy efficient upgrades are rolled into your replacement plan. Once you save money so that you can spend money, then you can focus on the big capital investments like an HVAC system.

Agree with the last comment that Lighting is definitely an easy upgrade  tat will impact your bottom line. Also with the new lighting legislation  act, between 2012 and 2014, standard A-line 40- and 100-watt  incandescent  light bulbs must use 30% less energy, but produce the same  light output as the incandescent bulbs most of us use today. There's  more information on  http://www.grainger.com/Grainger/static/rc_lighting_legislation.html.

Joshua and Tom have both provided great answers. Without knowing the size of your operation and energy costs I would only add that the data analysis is key but if you can find a local energy efficiency consultant to help you build a business case and prioritise the changes this will ensure you only do what fits your specific needs (ROI etc.). Unfortunately if you contact product suppliers (HVAC/light/heating/appliances) their focus is to sell you new product regardless. You need a more impartial view if you're looking to create an energy efficient environment.