There is one more method, the use of the "Dark Corridor". There are those who are on a fallen path, essentially not being on the path they should be. Only those who can be said to have a Dark existence or an In-between existence can make these doorways. On rare occasion those with particularly strong feelings or hatred, such as the case with Beast and Diz and perhaps others like them can open these paths. However you must be careful when coming into contact with such darkness. As such, if you use these paths too often you will be completely swallowed by darkness. Sora has used these paths several times before, but the degree of frequency hasn't allowed the darkness to stain his heart, so you can think of the influential power of darkness as being dependant upon the strength of the persons heart.
As for the Nobodies who have no heart and the King and Riku, using these paths many times doesn't concern them. Why doesn't the darkness progressively swallow them you ask? There are secret similarities between the two that are talked about in the game.