1) Pick a weekday game
2) Sign up for the fan club, it's the only real way for Joe Public to buy tickets from the club
3) If you're okay with going in without a ticket, the twitter account @arsenal_tickets is a great source and there are always a ton available the week prior to a match. They post tickets for sale by fans at face value. They have a pretty solid system in place & scampers get ratted out and banned.
I haven't attende a game at Emirates, but that is what the supporters tell me is the way to go. I have not found a good secondary market. Stub Hub exists over here, but it's pretty limited.