I ride alone for hours at a time in rural areas....

As previously stated:
1. Bring pepper spray.
2. Bring a cell phone
3. Self-defense knowledge is a good thing.

Additionally:
4. Bring enough tools to reasonably fix your bike/flats and have the know how to do it.
5. Get a road ID. If someone hits me and I end up dead/injured in country ditch, at least the police can notify my husband quickly instead of spending lots of time trying to figure out who I am.

Another cool thing is a product called the spot. It's one of the most affordable gps emergency devices out there. http://www.findmespot.com/Home.aspx