Is it mostly blue or red for example. Next, identify unique field marks such as a black head, white stripes on the wing bars, yellow belly or white spots. Notice the shape and size of the beak. Or is it long and slender like an insect eating bird. The beak is an important clue to the family and type of bird you're trying to identify.
Ornithologists organize birds into family groups that share certain structural characteristics.
Identifying family resemblances is often helpful in identifying birds in the field. A good example is the Picidae family which are quickly recognized as the woodpeckers. This would narrow your search from 800 down to 22. Behavioral traits also provide some good clues. This general classification step will make identifying the bird in your field guide easier.