Unless you know someone who's involved with the show you stand no chance of getting a ticket.
Tickets are ONLY available via ApplauseStore. I am on the list for X-Factor tickets, which is a reserve list only due to friends and family members of contestants and production teams, along with celebs, take them all.

I registered and about 3 weeks ago got an email inviting me to get tickets.
You could only have 2 tickets in total and you could only apply for 1 night on the list.
Tickets are then issued on a first come, first served basis. They would have been gone within 30 mins of the email going out.
Even if you get a ticket you're not guaranteed to get into the show...... it's a standby ticket, so only get in if they have space.

Naturally, we decided not to bother applying!