Naturalizing simply means that a bulb flowers again after the first year. Some bulbs are planted as annuals and will bloom only one season. All the food they need to flower is contained in the bulb. Naturalized and perennialized bulbs, however, need your help to get the food to recharge and regenerate for the next season's bloom.
Fall of the first season: When planting, work a good organic compost or well-rotted cow manure into the soil. Apply a layer as a top-dress or mulch, as well. Or, add compost or peat to the soil (for drainage) and top-dress with fertilizer: 9-9-6 NPK slow release or either 8-8-8 or 10-10-10 NPK fast-release soluble fertilizer (about one tablespoon per square foot). The slow-release method is, frankly, the easiest approach.
Come spring: If you used slow-release fertilizer, don't do anything. If you used fast-release fertilizer, apply a nitrogen-rich, fast-release NPK fertilizer just as the shoots first emerge from the soil. Fertilizing at bloom time or after can lead to disease issues.
Fall of the second season: Fertilize again by your method of choice.
Dieback: For all bulb plants, let the green foliage die back naturally after bloom for approximately six weeks. This is the time when photosynthesis creates and stores the sugars that fuel next year's bloom. Resist the urge to tidy up fading foliage or mow over it. An easy disguise is to interplant hostas, coral bells, lilies or other perennials that leaf-out early in the spring.
Deadheading: Tulips, yes. Daffodils and others that naturalize, no. Fading tulips will form seeds that sap energy from the bulb, so snip off the fading flowers.
Sometimes, naturalizing just doesn't happen. It could be the setting, the soil or the sun. Heavy summer watering can be a factor, too. And sometimes, it could just be the type of bulb. Some fritillarias and hyacinths, for example, just have a hard time making a strong comeback.
While planning your long-term naturalized bulb garden, think about adding a few stunning short-term, non-naturalizing bulbs in high visibility spots. You'll bring whimsy to your garden and be able to experiment with different bulbs year after year.