Make sure you can EASILY change your VTX channel. You will be assigned a frequency and will need to make sure you can change it without taking your quad apart.

MAKE SURE YOUR FAILSAFE IS CONFIGURED CORRECTLY. If something goes wrong, you don't want your quad to crash and start a fire, fly away, etc.

Setup your OSD to show your callsign on it. This is helpful when you're trying to find your quad's channel on your goggles and everyone is sitting at the starting line looking at the same thing.

Do a pass and make sure anything that might fly off is nicely secured. Your ESCs, your PDB's inductor/cap/buzzer (hot glue helps here)], your HD cam if you have one, etc.

Hot glue any wires that connect to your PDB, so they don't come loose and break. Your camera and VTX especially.

Loctite your motor screws and any other metal on metal connections.

Make sure you have spare VTX antennas, props, batteries, screws/nuts, velcro straps, zip ties, double-sided tape, electrical take, heatshrink, etc. Things will break.

If you're going to by flying over freshly watered grass, you might want to apply liquid electrical tape to exposed wires. Don't go crazy and cover your FC, but do cover the connections on your PDB and make sure your RX isn't naked (heatshrink it at least).

Charge your batteries for your quad, goggles, and TX :)