Make sure there's plenty of available storage space on your phone or camera before you film. It's always easier to take more footage and edit out what you don't need than try to shoot more footage after the fact.
Familiarize yourself with your device settings. Play around before you're trying to get clean shots to make sure you can handle any possible issues that can arise while filming.
Check your power levels. There's nothing worse than running out of juice in the middle of filming. Make sure your device is fully charged and, if possible, pack a backup battery.
Choose a single aspect ratio and framerate for all footage, especially when using multiple devices for filming.
Film with your phone held horizontally to avoid large black bands on either side of your footage.