Conservation often means to the public reducing or doing without and connotes pain and Jimmy Carter shivering in front of a fireplace with 15W incandescent bulbs, or reading the New Dork Times by flashlight.
Efficiency has a kinder definition, that in a way relates well to sustainability.
Do better with less, sure. Efficiency is looking at the whole process, and in so doing creating more opportunity for not only the reduction in energy use but the application of creative ways to do more with what we have now and what is likely to become less in the future. An efficient building or system can better utilize PV, wind, biomass, etc. in a cost effective way. It takes a little more effort to understand this as opposed to turning off stuff and feeling pain. Sustainability ties in when we seek to have our cake today, and leave some for future generations to eat too with out it shriveling or getting worm-eaten due to our largesse.