@Exogenous
You have a few options:
1) Write it down.
3) Have IMVU remember it.
3) Use a password manager.
4) Use 3rd party software to create a locked text file.
5) Roll your own security solution like I do (requires basic scripting and/or programming knowledge).
6) Memorize it.
All of these have advantages and disadvantages and have varying degrees of security. It mostly boils down to your personal requirements.