Use a strong passcode on your phone with digits and, if possible, letters.
Do not use biometric authentication like fingerprint detection (Touch ID) or face detection (Face ID).
Take backups or sync to the cloud to avoid losing data.
Beware that the government may be tracking your location with fake cell towers.
Consider purchasing a second "burner" phone to use for sensitive situations.
Considerations for when purchasing a burner phone.
Avoid putting your burner phone number in your real phone's contact list, and vice-versa.
Avoid syncing your burner phone to the cloud.
Avoid connecting your burner phone to the same WiFI networks and Bluetooth devices as your real phone.