These might be glaringly obvious, but I was tripped up by them early in my birding days. (Bear in mind that "early in my birding days" refers to a fairly recent time; I'm not the old hand that many of the people around here are.)
1) Keep the time of year in mind, when trying to identify a tough bird. Some birds look completely different, season to season.
2) Some birds resist identification because they're hybrids. I was utterly stumped by the Western/glaucous-winged gull hybrids round here, for ages. (Gulls are hard, to begin with, but these absolutely threw me for a loop.)
3) If you're having a dull birding day, and seeing only trash birds, it can be worth sitting down for a while and watching those trash birds. I've spent many a happy hour watching the strange things crows, gulls, and house sparrows get up to, when they think nobody's looking. Weirdest thing I ever saw: a line of five or six gulls on a rooftop across from mine, shifting from foot to foot in perfect sync, and bobbing their heads. It looked like they were dancing. Favourite gull behaviour: the foot-stamping worm dance, where they run in place on the grass to get worms to come to the surface.