I have been deep in previous seasons, and not been successful.

Here are my reflections.

When submitting an application, my advice is "set and forget"

If you get an interview you need to consider these things in my humble opinion:

1. Market your strategy/game play to them so they can clearly picture you in their puzzle of casting. Know your strategy inside out, they will try to catch you out, call your bluff. You have a seat at the table, don't waste it. Be ready and prepared.
2. Be authentic, unless you are a bloody good actor. If you bullshit, they will pick it.
3. Realize that luck is a huge part of this. You could have executed your plan superbly but your puzzle piece just doesn't fit. They are looking for only 12 people out of 35,000 applicants, and half aren't your gender and half aren't your age demographic. In reality, you are up against thousands of applicants for maybe 1 or 2 spots that fit your demo. Most seasons will feature champs now, they are here to stay. Even if they theme series, they will have champs attached to the brief. Champs rate. The odds are not in our favour.
4. If they have seen you before, chances are you wont be seen again. Move on, be happy. They are ruthless, but have to be in fairness in trying to reach as many applicants as possible. Its only fair that way.
5. Endemol do a great job, the show is amazing. Its easy to get cynical but the reality is So many people want to play, but so many people wont. So many people are worthy of playing, there just aren't enough spots.