Short Sessions

This is a small article, but it describes a key concept whose importance cannot be overstated:

Keep your sessions short so that you're always training at your dog's peak motivation.

For a young dog, you might let the dog have two or three retrieves, then take a break.

For an advanced dog, a schedule similar to those used by many of the pros might be a good guide: Training days separated by one or more rest days, each training day consisting of perhaps one land series and one water series, the series separated from one another by hours.  Such a schedule might not be convenient for the amateur, but it's an ideal to aim for.