1.- There's a mini game at the ship that takes you to Lucca at game's beginning, when you go from Besaid to Lucca, Tidus will remember Jecht and his past, and then he will start to shot over and over again, try to take a good score to get this shoot, you could even have it yet and you haven't notice (this happened to me, haha), and I think so, only use Tidus's first Overdrive with weak monsters (because is the one where gray line goes slow, and you can hit it easily)
2.- Don Tonberry is a special monster in Monster Arena, to get it, you need to catch 1 of each monster from Cavern of the Stolen Fayth, and the trick is in another topic in this forum , so I will try to not take it again because I don't want to get in troubles
3.- Half way, the boxes-like monster transforms into a shadowy monster and if you defeat it, you get around 50,000 gil, and with Gillionair, you get 100,000!

Don't give up at Blitzball, I hate that game in the start, but right now I can take around 12 shoot over the enemy easily (this sound like television commercials, sorry)