Finevol, my suggestions for handicapping the Derby:
1) Be aware of the running style and tracks over which the contenders have run. This is because you need to account for the long distance of the Derby (1 1/4 mile is a lot for 3 year olds and most will not have run more than a mile prior to this race) and it is over Churchill Downs, which IMHO is not a speed-favoring track.
What I mean by that is that some tracks are speed biased and the layouts (the shrapness of the turns, that kind of thing) favors horses that run on the lead. The main examples are Gulfstream Park and Santa Anita. Horses that ran well on the lead there may look deceptively good compared to horses that only made up moderate ground at the end of the same race.
For example, a horse that wins the Santa Anite Derby from on the lead or just off it is probably not going to fare nearly as well at Churchill at 1 1/4 miles as a horse that closed from 8th to 4th or 5th. The fact that there will also be a much larger field than in any other race adds to the benefit of closers IMHO because there is usually a battle for the front and that tires out the horses on the lead.
2) The favorite rarely wins the Derby. Just a fact. With 17-20 horses, traffic trouble and four other young horses simply "waking up" and running their best of the season is historical fact. Sure, the favorite will sometimes win or place in that race, but over time if you want to eek out any profit don't bet the favorite at the Derby.
3) My particular style in the past has been to pick the best closer that is at least 15 to 1 and bet him to win and then wheel him second in the exactas plus a couple of extras with the other horses I like. It gets expensive because of the size of the field, but its a race where there is a genuine opportunity to get a nice price back on horses that are clearly talented and whose owners and trainers think are hitting their stride.