I've had a bit of success backing 2nd and 3rd pre-post favorites from the Winning Post at Sat Metro. as long as their SP is above $3.50. It would probably work just as well from any other form guide source.

I put in the filter that must have started within 21 days and were not 1st up.
Works like a rocket some days but can be tardy on others. Long term, I wouldn't have a clue how it performs as I've never used it every week over a decent period of time. It certainly catches quite a few winners though.