There's a lot of things to look out for when it comes to horse racing.
1) Don't always go by the form, you could have a horse thats won 4 out of its last 5 races, but it could have been racing total donkeys.
2) Look out for drops/increases in trip and classes.
3) Trainer/Jockey combinations. Prime example of this was at Newton Abbot a few weeks ago, Philip Hobbs and Richard Johnson usually do well there, first race, short-priced favourite, and this horse was 25/1. I pointed it out to my dad, and sure enough, it won.
4) Take note if its raced against any of its opponents on the day in the past, the going, trip, etc.
Probably loads more i've missed but i'm nearly asleep at my keybaord, will update it tomorrow