If you don't have a dog (or don't have a good dog), look for small areas that would channel birds to somewhere they'd want to take flight (open fields, etc.); and walk and stop. If you pause every so often, it works to make birds nervous so they won't just hold while you walk past.
Get practiced at bringing your gun up fast to surprises... it's easy to think nothing's coming after a some long walks. I shot over two birds this year because they came up when I wasn't expecting them and I didn't shoulder the gun properly because I had almost no time to shoot.
If you have a bad dog, leave it in the truck. I went hunting with someone one time with an undisciplined dog and got to watch it flush birds 100 yards away that we could've walked up on and taken otherwise.