I believe that Monkeysystem wrote an article for bjinsider on secret bets a few years ago. Maybe he will post a link.
The one general rule of thumb that most agree on is to save it for the final hand, or for making a big move near the end of the round.
From my experience, you can almost always assume that folks using a secret bet will bet big. This is especially true for non-experts. As always, you need to asses your opponent's skill and tendencies. I, on the other hand have had great success in using the secret bet to take the low on players who are ahead of me. Once again, be careful, as this can lead to being locked out!
In general, I like secret bets, since I believe that they represent an opportunity for skilled play.