It's a myth. I even believed it, and everyone I know that takes LSD still believes it, too. It's just really good DEA propaganda to slow the traffiking of LSD and to stop the uninformed from consuming lots of it.

Quote Originally Posted by Erowid LSD vault
LSD does not form "crystals" that reside in the body to be "dislodged" later, causing flashbacks. LSD is a crystalline solid (though it is unlikely that one would ever have enough to be visible to the naked eye) but it is easily water soluble, thus cannot form bodily deposits. Furthermore, it is metabolized and excreted in hours. The bogus "loosened crystal" description in not necessary to explain flashbacks, which are psychological phenomena (see FLASHBACKS).

LSD does not cause chromosome damage.
And then.. when you go the Flashbacks section:
A simple explanation of LSD flashbacks, and of their changed character after 1967, is available. According to this theory, almost everybody suffers flashbacks with or without LSD. Any intense emotional experience--the death of a loved one, the moment of discovery that one is in love, the moment of an automobile smashup or of a narrow escape from a smashup--may subsequently and unexpectedly return vividly to consciousness weeks or months later. Since the LSD trip is often an intense emotional experience, it is hardly surprising that it may similarly "flash back."
So basically, do not take more than 1 or 2 hits of acid. This way, your experience won't be that detrimental psychologically, and in turn, your chances of having flashbacks are greatly reduced.