Only time I went to Del Mar, with some AW.com peeps, I used the train. It was super easy except for the train being delayed due to Comiccon opening on that same weekend.
If you do take the train, they have buses at the station waiting to take you to the track. So it makes it super easy. On the way back, you get on the bus, and it takes you right back to the station.
Oh, and Downing, as for betting. Bet on horses that have done well at Del mar in the past. Regardless of how poorly they are doing now. Del Mar is a funky track for horses.