I usually ride from home, but sometimes haul my bikes in our 2004 Prius. Fold down the back seats, remove the bar/cover assembly, take off the front wheel and slide the bike in through the hatchback. It is easy, cheap, and very aero. Put down a tarp if you got you bike muddy on the trail.
If you want to carry two bikes or more you are going to need a rack. Coastal Tech has a reciever hitch which will let you install a hitch mounted rear bike carrier. This type of carrier might be the most aerodynamic but you run a greater risk of getting you bikes smashed in city traffic. Roof racks can be hard for some people to use, especially if you have heavy bikes. If you have a garage, and your bikes are one the roof, put the garage door opener where you can not reach it from the drivers seat.
Your local bike shop would be a good place to inquire about bike racks. They know what they are talking ahout.