Keep the area around your house clean. Arson is often a crime of opportunity. Arsonists will used dry leaves or branches, old mattresses, bags of leaves or garbage, chair pads from lawn furniture, and newspapers to start their fires. It lessens the need to bring materials with them, which means they are less likely to be noticed.
Know your neighbors. They will be more likely to keep an eye on your place and you will be more likely to know who doesn't belong in your neighborhood. Question strangers to see if you can help them find who they are looking for. Call 9-1-1 if you have any indication that they should not be there. Call 9-1-1 if there are other suspicious things going on, like people moving around at night or carrying containers into houses or structures.
Install smoke detectors on every level of your home. Often, the smoke detectors are the only warnings to get out of a burning building.
Finally, if there is a fire, pay close attention to people and vehicles in the area. This is especially important if they aren't typically in your area. Note things like gender, age, hair color, height, race, clothing, and vehicles.