Overall Energy Use
Buildings account for about 40% of overall energy use, the rest going into transportation, industry, retail and offices. Of the transportation amount, a huge percentage of it is to move goods rather than people. If we are to reduce, or eliminate fossil fuel use, we will need to attack these other areas as well.1
Travel
No discussion of carbon emissions is complete without looking at travel. Once you live in a reasonably efficient house, it is very easy to use more energy (and carbon) in transportation, and if the house is very efficient transportation will dominate your carbon footprint. For example a new reasonably energy efficient house might have a total energy use of around 60Mbtu (around 18,000kwh), which is the equivalent to around 500 gallons of gasoline (at 120kbtu/gallon), while a super-efficient house might only use 20Mbtu a year and a super-efficient multi-family unit might use only 10Mbtu/year.
Assuming that you drive the standard 15,000 miles a year and get the typical 25mpg, that amounts to 600 gallons: more than the entire house. Even the gas-sipping Prius at 50mpg uses 300 gallons a year: a significant addition to your carbon usage.
For someone who flies, you also have to account for the airplane's approximate 50 miles per gallon per passenger average-- a 3000 mile cross country flight uses 60 gallons of gas per passenger, or 120 gallons per round-trip cross country flight. Trips under 500 miles get less than 50mpg/passenger because the take-off and landing use significantly more energy than flying thru the very thin air above 30,000 feet.
Trains are better, but not that much better, particularly since the often operate far below their most energy efficient capacity. High speed rail, particularly above 100mph, and especially above 150mph, requires an increasingly large amount of energy to go the same distance.
Intercity buses can get up to 11mpg for the entire vehicle, so a bus with 20 people on is achieving a very respectable 200+mpg/person. In city buses, because they have to stop so frequently are not nearly as efficient, typically getting somewhere between 2.5 and 3.5mpg. With that same 20 people on board, your hardly doing better than a single occupancy Prius. A vanpool van at 10-12mpg with 6-8 people aboard does just as well or better than most city buses.
Stuff
Most goods have traveled far before you buy them. Even food has often traveled over 1000 miles before you get it. The simple answer is to just buy less stuff, but of course there are social implications to that. Other good solutions are to buy very durable things and maybe even buy them used, buy stuff made locally or out of local materials. In particular buy food grown locally.
Notes
1: there are now a lot of good books analyzing how to reduce energy use. One I ran across that seemed accurate and reasonably objective was "Heat" by George Monboit.