There was talk of something which would make it easier to find old comments but nothing's happened.
In lieu of that, you can copy (via the Share option) the link to your comment and paste it somewhere else. On MDL that could be in the Notes section for each title or a Custom List. Off-site, a personal blog or just a note/text file on your device. I back up copies of some reviews off-site for myself, but there's no reason why it couldn't be comments. Copy/paste the link + what you wrote - will be easier to track down something you half remember in future years :D
