My advice is to just play... your characters will become great in no time. Blitzball is no challenge, just train your characters and don't get new ones because your characters will be so much better then everyone elses (worked for me)

My strategy...
Have only Tidus shoot (Jecht Shot)
And Have everyone else have good defensive techniques (Drain Tackle, etc.) and good passing techniques because Jecht Shot can and will win any match for you.