What are the best ways to keep someone out of my device?

[AC Question]

It may be impossible, but I want the most effective way to keep anyone from accessing my accounts and profiles from old devices.

 

[Cthugha]

Your current device? Set up a password or other security check (usually settings -> lockscreen). The fingerprint reader on my Galaxy S6 is quite nice

On old devices that you're not using any more? Wipe them or at least remove your accounts when you're done with them. If that's too late you can remote wipe, delete the device ID or change your account password (i.e. Google).