Couple things (assuming you are in NA):
-During breeding season, you can try playing a call on your phone. Works very well with most spring warblers (but not all; in fact playing a call can send some birds 100 ft up into the treetops to sing)
-Pishing. Use when birds are hiding in brush. Works great with sparrows year round. It will also make chickadees come and check you out (if they weren't already). Also works with some scrub-nesting warblers (like the common yellowthroat)
-Knowing what you're after and behaving accordingly. As some other anons have already pointed out, experience will help you get close to birds that are more spazzy. Knowing when and how to be sneaky.
-Staking out a good spot. That branch you sometimes see a kingfisher on? Go sit by it for a while and try to be inconspicuous.
-Luck. Sometimes a bird just lands by you or just doesn't give a fuck that you're lurking nearby. Often the case with certain migrating fall warblers, in my experience. The photo is one of my favorite American kestrel shots I ever got. They 99% will fly off when you get anywhere close, but this dude was the 1% who decided he just didn't care.
Hope that helps