I have a few rules for selecting races and horses.
* You must have a reliable rating system to rate the fields. I use a personal spreadsheet based on times - subscription services or Techform are OK as long as they are consistent.
* Stick to sprint races only (<1600m). Times are almost meaningless in staying races.
* Stay away from wet tracks - slow or worse.
* Eliminate 2YO races. Too young, too green and not enough exposed form.
* Convert your ratings to prices, as a bookie does - total market = 120%.
* Back the value bets, where there is a substantial margin between your price and the market. One bet, straight out, per race - no each way or place bets.
* Best tip of all - record all your bets in a spreadsheet and analyse the results - continuously.