cameron398, just my 2cents worth, 2 rules that I have found invaluable as filters for favorites or any other selections are as follows:

(1) Don't back any horse that that finished more than 3.5 lengths from the winner last start.

(2) Don't back any horse that is set to carry 3.5 k'gs up or down from it's set weight last run, (ignoring all allowances)

As a footnote if backing for the place, a horse that has run within 100 metres of todays distance at least once in its last 3 runs, and one with at least 50% place S/R DEFINITLY lifts S/R BUT depending on the method of selection may also result in a drop in POT so what Mark says has some relevance as well!