Last year they were selling weekend tickets on the door for face value. If that fails, theres usually loads of touts sitting around the entrance who will have tickets, but check the door first. (There were wardens with loudspeaks telling everyone not to buy tickets off touts, as there will still some left on the door..)
If your feeling risky, try ebay literally a couple of days before the festival starts, and either arrange to pick up the tickets from somewhere, or get them sent special delivery. My mate managed to get one for £115 (less than face value) off ebay at the last minute last year, but it was sketchy as it got delivered the morning of the festival.. (by which time he'd already left and his girlfriend had to pick it up from the post office - not something id like to do:S)
Anyways, ill be going, should be fun. Glastonbury first though, woop woop!