The KEEP command will MARK a memo as being kept. Then the memo will not expire unless deleted. Deleting it either by number or in a del * command will remove the memo.
/msg memoserv keep 3
Memos that are read but not marked with keep will expire whether you delete them or not. You should still delete and expunge your excess memos to keep your memobox easier to go through and to reduce using up service's space needlessly. If you have any important memos you will want to be able to reference later, mark them with keep. If you find yourself doing this often, you should probably make a folder for such memos, so that you do not accidentally delete an important memo with a del *.