How to keep your seed phrase properly : Write it down in a note , and keep it in safe place (safe from physical attack like water , fire etc) . And put it in a place you can easily remember or access, so that you won’t forget where you put it , because if you lose it you won’t able get it back .
It is quite interesting that I have had few experiences, especially from people's mistakes. One major concern when it comes to the man handling of seed phrase is people's inability to adher to instruction. Because I want to believe that most mobile wallets that we have these days does few things that teaches some basic level of privacy to the user. The few things AFAIK...
- It doesn't allow screenshot of seed phrase, which should be enough reason to raise one's curiosity
- It informs a user to write down each word produced, and some wallet go as far as dropping the word one at a time.
- It also request for few words out of the 12 for verification purpose at that instance.
And I guess some wallet might emphasize the importance of those words in a very short sentence.
To backup seed phrase online is not advisable.
Well, it's good that the OP corrected the statement, because I have seen a Gmail hack, and the victim resorted to keeping his phrase there before the hack happened. It is clearly not a secure means of storage.