Size matters. You should pay attention to how big or small the bird is. You can use size relativity to get a general idea. Size relativity refers to being bigger or smaller than a sparrow, robin, crow, or pigeon, per say.
Study silhouettes. The silhouette is an important part of identifying it. When it is flying and you can't see all of the details, just pay attention to its body, beak, tail, and legs. Ask yourself these questions:
Is the body short or long? Narrow or plump?
Is its beak fine, long, or short and stout? Is it dagger-shaped, hooked, or straight?
Is the bird's tail rounded, square, pointed, or forked?
Are its legs short or long?
Do the wings look rounded or pointed?
Do the wings have wing bars? If so, are they single or double? Bold or obscure?
Pay attention to the bird's behavior. Different birds have different behaviors. For example, when woodpeckers climb up a tree, one might climb in spirals, and the other might climb in jerks. Also, they might climb using their tail as a brace or headfirst. If you know what a bird's behavior is like, this can be a helpful tool in identifying.
Take note on where you saw the bird. There might be a bird that looks exactly like it, but is on the whole other side of the world. Range is a very important part in identifying birds.
Don't depend on color. Birds have different plumages at different times of the year, so the color of their feathers might not tell you what kind it is. You should use color as a last resort.
Use a field guide. It is best to use a field guide last, because you will have all the information recorded when you go to look it up. Painted pictures in field guides are often better than photos, because photographs sometimes have lighting problems. Most field guides have categories that the birds are put into, or they separate them by family.