One of the keys to having a healthy cockatiel is to feed it a nutritious diet.  The three main parts of Kramer's diet are seeds, greens, and vitamins. 

Kramer seems to enjoy the prepackaged cockatiel seed mix we feed him.  If your bird doesn't take to the prepackaged stuff, try making your own mixture tailored to your cockatiel's taste.  Start with a basic seed as your base, and add dried vegetables such as broccoli and carrots.  Before adding any dried up plants, please read the toxic and safe plant lists prepared by the National Cockatiel Society (links are below), as there are quite a few plants that will make your cockatiel very sick.

Because Kramer was "mating" and laying eggs without a male present, our vet suggested switching her to a half seed, half pellet diet.  Although Kramer really would prefer just a straight seed diet, she's getting used to the mixture we give her.  Just remember to try and keep your cockatiel's diet as stable as possible- don't go giving it a different seed everyday just for variety. Only do so if your vet tells you to.