Here is a system that I have found on the Internet, and which I have adapted to Australian racing conditions.

Rule 1: Operate only on handicap races.
Rule 2: A contender must have won by more than one length in its last start.
Rule 3: Eliminate any fillies and mares from consideration.
Rule 4: A contender must have had its last start within the previous 21 days (inclusive).
Rule 5: Must have started at 10/1 ($11) or under at its last start.