I bought a ticket a couple of years ago outside of White Hart Lane from a season ticket holder - even sat next to him (he had two tickets and was selling one). Did it in Barcelona as well. There were loads of people selling tickets outside the stadium before the match and the closer to the start time, the cheaper they get. If you're willing to risk not getting in, this is probably the easiest way, and if you don't make it then go to the pub nearby to watch. Similarly when I went to see Billy Joel at Wembley a few weeks ago there were touts outside the stadium selling tickets for less than what I paid on seatwave.