Given how long it takes to master elements, Team Avatar should have realized they probably couldn't make much of a difference in Book 3 just by training bending abilities during the short amount of time they had left. Why didn't they try anything to reconnect Aang with the Avatar State? As it was proven repeatedly, it's the best advantage they had against the Fire Nation.
Aang was basically fixed by getting hit in the back with a pointed rock. They could have tried to find the guru again, and maybe he would have given a massage or something that would have brought back the Avatar State. In the discussions preceding Sozin's Comet, the characters hardly mention this superpower, the only real chance Aang had against the Fire Lord.
Why didn't Aang and friends make any effort to bring back the Avatar State? The way the writers did it really seems like a deus ex machina. It meant that Aang's victory hinged on the small chance that he gets slammed into a rock in the right way. Not only was the guru alive, but Aang also had the wisdom of all past lives, some of whom might have had the knowledge to help.