+1 on a mirror. I try to stay off the heavily traveled roads as much as possible, however, sometimes they cannot be avoided. I try to watch what is coming up behind me and what is coming toward me from the front...timing all the approaching vehicles is important. Sometimes I give them a break and stop on the shoulder and let them pass. This is especially true for two opposing Semi trucks that by my estimate are going to meet with me at the same time. Other times I take my lane (watching rear traffic closely in my mirror) until the front approaching traffic is clear. By taking my lane, I mean I move to the left just enough to encourage rear traffic to take notice and hopefully begin timing their safe passing path/timing around me. However, there are some idiot drivers out there, so...be prepared to get back to the right quickly. Some drivers forget they even have a brake, and are incapable of slowing for cyclists, those are the ones I watch out for.

Staying off of the busy roads is definitely the preferred method.