A new feature in Kingdom Hearts II, is Sora's Forms. These Forms allow Sora to merge with the power of another player, and increase his combat skills. Each of these forms takes up a certain amount of Drive Points. While Sora is in a form, the Drive Gauge becomes a Form Gauge. As you increase your experience in a form the guage becomes longer lasting, thus allowing you to remain in that form for a longer period of time.
As you can see from the image on the left, the Drive/Form Gauge appears right above Sora's MP bar. When it is full, as it is here, it will show a MAX over the gauge. This means you can then change into a form, that consumes the same amount of Drive Points (or less). Once you change into a form, the gauge will begin to deplete. If you gather Drive Orbs while in form, it will increase the gauge, but not by a lot. However, abilities such as Drive Boost help make keeping the gauge full with orbs easier.

Leveling Forms

With the different ways that you level each form, it can get very confusing as to where the best place is. Well, I scoured every world to find the perfect places...well place. Of all the locations, and all the guides, I found one place to be the best. And that is Beast's Castle. When leveling Valor, Master, and Wisdom Forms, Beast's castle provides the best place. There are enemies a plenty, making the Valor Forms rule of striking enemies good. About 95% of the enemies are Heartless, making the rule of defeating Heartless for the Wisdom Form easy. Finally the vast majority of the enemies drop Drive Orbs. This not only makes leveling Master Form a breeze, but it also provides the greatest place to get the Drive Gauge back up, without leaving the world. Only the Final Form cannot be leveled in Beast's Castle very well. This is due to the fact that the appearance of the Nobodies in Beast's Castle is random. This makes it difficult to predict where to change to Final Form, without a specific room that Nobodies are in. But fear not, I know the best location for that as well.

Since you have to defeat Nobodies to level the Final Form, there is only one place where they are found in abundance...The World That Never Was. In the Organization XIII's fortress, locate the save point and Moogle Shop in the "Twilight's View". From here, go onward to Naught's Skyway. These few locations provide the most Nobodies of the entire fortress, Naught's Skyway providing the most. I simply changed to Final Form in Naught's Skyway, and worked my way back to Twilight's View.
With leveling the forms in these locations, I think I spent a total of an hour and a half leveling all four forms. I utilized the Drive Boost ability as well. With this ability, once I reverted from the form I was leveling, I cast Cure. Drive Boost almost triples the rate orbs fill the gauge while in MP restoration. This made the drive recovery super quick.
Also I did wait until Xaldin was defeated in Beast's Castle, before I did the venture there as well. But this is of course not the only places to level your forms up. Find the right place for you.