FNG is spot on about having qualified people do your pyro for you, especially if there's invitees playing. It will cost you a little more but they carry liability insurance and have training and experience to make sure people and property stay safe. Very important considering how litigous America has become.
Whiny punk: "Ouch I got an ember in my eye and I'm blinded for life!"
Judge: "The defendant is ordered to pay damages in the amount of 25 million dollars."
Established objectives are also critical. A plot isn't really necessary but definitely helps out.
Props are a huge bonus, especially functional ones. I picked up a 4-channel radio remote control relay box for $30 CAD, has a small keychain remote with four buttons and a 12 volt relay board with antenna. Hook up a battery and wire up a simple circuit, and you instantly have remote controlled devices of all manner. Anything electric powered can be controlled by it such as mines, alarms, lights, noisemakers, directional cannons, etc.