I totally agree with the others here that a good system should have a few rules, 4 or 5 max and these rules must be logical. If you do the same as everyone else is doing regarding filters then surely you must lose though.

An example of an all too often used filter in a system is "days last start". You get the impression that almost everyone uses this filter, let's say it's Must have run within 21 days. All those horses that have run within 21 days (which incidentally will be a lot of the field) will be carrying far more money than they should compared to their "real odds / chances", and hence they'll be under the odds. Another example is Must have run within the first four at it's last 4 runs ..... A system has to have limits doesn't it, BUT, just to be different, and get value, why not allow the horse to have one of it's finishes to be 5th or 6th ? That will set you apart a little and give you a chance of some value.