I don't have a "system" as such but the "rules" for me are looking like:

*Only metro races, weekends, public holidays etc.
*No sprints or hurdles.Prefer 1200-2000m races.
*No more than 14 runners
*No 2yo, maidens etc.
*Zip diff. (see above) of less than 2.0.
*No races with two Zip starred horses

This still leaves lots of races but hopefully weeds out the tricky ones. Any more filters and the number of eligible races starts to become a problem. Sometimes I eliminate races with more than two last start winners as well but this cuts out some good races. The problem is to find highly specific filters that don't impact too severely on volume of races.