The memory feature on LiveJournal allows you to mark your favorite entries from LiveJournal in an easy to use keyword system.
The easiest way to set memories for your own account is to enable your message board system. ( Refer to : http://livejournal.com/support/faqbrowse.bml?faqid=62 for more detailed instructions. )
In the Read Comments page is a small toolbar that offers a variety of buttons, one of which is a small red heart, clicking on this button will take you to the "Add to Memories" page. This toolbar will only appear if you have set up your message board system.
On the "Add to Memories" page you can give the memory a descriptive name and associate with it up to five keywords or phrases, each keyword or phrase must be separated by a comma (for example: "tarot cards, tarot, reading, fortune telling, gypsy").
An alternative method is to go directly to http://www.livejournal.com/tools/memadd.bml?itemid=XXXXX replacing XXXXX with the itemid number, this is not the recommended method of setting memories but it the only option if you do not wish to enable your message board system.
Remember that you are not limited to just the posts that you make on LiveJournal; you can also define posts written by other users as memorable, by following the same methods above.
Memories also come with 3 seperate security levels, similar to your journal entries. "Public" specified memories will allow everyone to see your memories, "Friends" specified memories will allow only the users named on your friends list to see that memory, and "Private" specified memories only allows yourself to see your memory, provided of course that you are logged in.
Editing memories is easier, if you follow the "Memories" link from your userinfo page, or by visiting the following URL, provided that you are logged in : ( http://www.livejournal.com/tools/memories.bml ). To edit an entry, find the memory you want to edit by following the keyword links, and then click on the  link next to the memory itself.
To delete a memory, simply erase all descriptions you have given the memory, and then click on the 'submit' button.