I'll just expand a little on what
chesaka and
PB said.
First off it isn't a given that every retriever has a good nose; some are distinctly better than others, and of course it varies from breed to breed. So you need to acknowledge that you've got what you've got, and the job now is to maximise on it.
The dog needs to have a cue for a trailing hunt so you need to work out a verbal command that he'll cotton on to even when no shot has been fired or bird has been seen to drop. I use "Seek on" just because it sounds a lot different from the others. It's also handy to have your dog nice and sharp on his turn whistle; that way when you are working a bit of cover out, you can put him into the likely places.
When you start the trailing exercise, make sure that the wind is in his favour, right up his snitch, and he's following a genuine game scent not your own foot scent that you laid when planting the bird. I use a dead bird on a length of twine round a tree so I can be well to one side of the drag. I'm sure you can work out something similar to suit your own circumstances. Cuing with feathers as suggested is a great idea and replicates what he'll contend with on a hunt; a point where the bird hit the deck, lost a few feathers and legged it. This is called "the pitch" in Britspeak.
Another useful drill is to use a hand launcher to fire the scented tennis balls into undergrowth. This has the advantage of putting a powder smoke smell on them, again replicating real life. Advancing from there and laying much a longer trail, use the launcher to fire one of the round semi hard PVC dummies along the ground. With a good blank you can punch one of these seventy or eighty yards through light brash; this is SOP with Springers. Pic as below.
Eug