Given the visual design of the sanity tokens, they seem more like INsanity tokens.
With that in mind, we've been adding them (rather than taking them away) from the cards of the afflicted investigators. Used in this way, each one serves as another disorienting blow to the fragile psyche of the investigator in question, driving him or her ever closer to the ravening maelstrom of madness.
Excellent approach. I'm surprised this isn't the official procedure.