How do you get the secret ending with tidus and yuna talking at zanarkand.
Also...
How do you get the mascot dressphere?

You get the Perfect Ending by getting 100% completion in either one game or through multiple playthroughs and within the game you trying to get it, complete the meeting with Maechen in Chapter 3, hear whistling and be led out of the Farplane it the end of Chapter 3, complete the Follow the Moogle mission in Luca in Chapter 5 (Episode Complete), get Episode Complete in Besaid in Chapter 5 and use "X" to call for the Fayth at the end of the game in the scene immediately following the "Chapter 5 Complete" tag.

To get Mascot, you need to get Episode Complete in every area possible (including Bevelle, despite what some guides say) in one game. In other words, if you got Episode Complete in most sections in one game and then started a New Game Plus and got Episode Complete in the remaining sections, but not the others, it would not work. It has to all be within the same playthrough. Oh, and don't make Zanarkand the last section you do, otherwise the game won't auto-return you to the airship and you'll get screwed.