Turn on Apple Find My or Google Find Hub before your phone goes missing, so you can locate it and lock it remotely.
Use a strong passcode, enable fingerprint or face unlock, and turn on two-factor authentication for your important accounts.
Enable automatic backups for your photos, contacts and messages so you can recover important data if your phone is lost.
Use Find My or Find Hub from another device to locate your phone, make it ring or lock it remotely; erase its data if recovery seems unlikely.
Contact your mobile operator to block your SIM, change important passwords, sign out of the missing device and alert your bank about potential payment risks.