Charge your phone ahead of time. Your phone battery is probably the last thing on your mind before the big moment. Plan ahead and make sure you've got plenty of juice to record your reveal.
Turn your notifications off. Whether it's a loved one or even more likely, a telemarketer, you don't want an incoming call to interrupt your song.
Put the camera in a sweet spot. Place the camera in a stable position to avoid shaking or movement during the reveal. If you're filming in your car, try using a dashboard mount.
Record the entire reveal. Hit record before you start playing the song. The little bits of conversation before and after the song really make the moment event more memorable.
Be natural. Finally, be yourself and let the moment unfold naturally. We aren't aiming for cinematic masterpiece. We're aiming for an authentic and heartfelt reaction from your loved one that you can cherish forever.