You need to setup your decoys where you've seen ducks sitting, feeding, resting before. The wind needs to be from your back. Ducks and geese will almost always land directly into the wind.
Search on the web for different decoy layout shapes. A roughly "J" shape with the long end of the J diagonally our curving in toward you has worked well for me. Keep the closest decoys within 10-15 yards. Leave enough open spaces for birds to land within range.
Don't overcall, especially when ducks are in closer. Let them want to come in and land. A mistake / off sounding call can send the ducks (or geese) away as fast as any movement. Cover your face with face paint or cammo cover. I don't even like to look up a the birds directly, until I'm about to get up to shoot. Obviously, you need to know where they're at... Just know they can see and hear you better than you can them. If you're going to hunt out of your boat, it's got to be well hidden, too.
Have fun!