I asked the very same questions to a couple of very experienced Proffessional punters many years ago.
In order of importance were

A/ Placed 1-3 Last start.

B/ Beaten lengths - 1000 - 1600 beaten no more than 2 lengths

1650 - 3200 beaten no more than 3 lengths

C/ 1st 4 bookmakers OPENING prices

Cheers.
darky