Lift and divide daffodil clumps just after they've finished flowering, to prevent them getting too overcrowded. Left to themselves, bulbs multiply to the point where they've no longer got the room or resources to flower, resulting in 'blind' clumps which fail to flower, so keep an eye on your daffs and if any are showing signs of feeble or non-existent flowering it's time to sort them out.
Using a garden fork to avoid damaging the bulbs, lift the whole clump up out of the ground and carefully split it into single bulbs using your hands. As you do this, give each bulb the once-over, discarding any which are showing signs of rotting – keep an eye out for the fat grubs of narcissus bulb fly, too, which will eat their way through your entire stock if you let them.
Before you replant, dig over the soil and beef it up with a good generous spadeful of well-rotted manure or other organic matter, adding some grit if your soil is on the heavy side. Then pop the bulbs in about 5cm apart, twice as deep as the bulb's height, and you should have a healthy and much expanded daffodil display for next spring.