Here is a system I have been playing with today.

It is based on the horse being the fav 3 starts ago and winning, and then being rated poorly for the current race.

Rules are :

1. Horse must have been Favourite 3 starts ago.
2. Horse must have won that race by 2 lengths or less
3. Unitab rating must rate horse at 95 or less.

What we are looking for is a horse that performed well when it was a fav. Since then it has probably had a couple of bad runs(unlucky, wrong distance, etc), or it is perceieved that it is out of its class this race (determined by unitab rating of 95 or less).

So far I have only tested 2 months as I grab the data from unitab but it has shown the following :

1st Month
Selections : 133
Winners :22
Win $ : 274.80 (107%)
Place $ :135.00

2nd Month (not quite complete a couple days still to grab)
Selections : 120
Winners : 17
Win $ : 192.3 (60%)
Place $ :118.00

I'll post whether this continues to work later tonight as I get other months data.

Good Luck.