How do I prevent accidental rm -rf /*?

One of the tricks I follow is to put # in the beginning while using the rm command.

root@localhost:~# #rm -rf /

This prevents accidental execution of rm on the wrong file/directory. Once verified, remove # from the beginning. This trick works, because in Bash a word beginning with # causes that word and all remaining characters on that line to be ignored. So the command is simply ignored.

OR

If you want to prevent any important directory, there is one more trick.

Create a file named -i in that directory. How can such a odd file be created? Using touch -- -i or touch ./-i

Now try rm -rf *:

sachin@sachin-ThinkPad-T420:~$ touch {1..4}
sachin@sachin-ThinkPad-T420:~$ touch -- -i
sachin@sachin-ThinkPad-T420:~$ ls
1  2  3  4  -i
sachin@sachin-ThinkPad-T420:~$ rm -rf *
rm: remove regular empty file `1'? n
rm: remove regular empty file `2'? 

Here the * will expand -i to the command line, so your command ultimately becomes rm -rf -i. Thus command will prompt before removal. You can put this file in your /, /home/, /etc/, etc.

OR

Use --preserve-root as an option to rm. In the rm included in newer coreutils packages, this option is the default.

--preserve-root
              do not remove `/' (default)

OR

Use safe-rm

Excerpt from the web site:

Safe-rm is a safety tool intended to prevent the accidental deletion
of important files by replacing /bin/rm with a wrapper, which checks
the given arguments against a configurable blacklist of files and
directories that should never be removed.

Users who attempt to delete one of these protected files or
directories will not be able to do so and will be shown a warning
message instead:

$ rm -rf /usr
Skipping /usr