My ticket is 'checked' most mornings at Waterloo East station. I have only been stopped twice in the last three months despite my ticket being completely unreadable due to the text being worn out. I would get a replacement ticket but I'm on my third this year and feel that:
a) tickets shouldn't wear out within six weeks
b) ticket inspectors should be stopping me on a daily basis rather than less often than the ticket takes to become illegible

What was additionally helpful was that when I was stopped in September the ticket inspector helpful wrote on the back of my ticket 31/12/11 E9749, so that other inspectors will know the expiry date on my ticket without putting it through their machine. It worked very well when I was stopped last week. I'm not sure what would stop somebody else writing the same on the back of a used ticket.

Station staff, even the revenue protection officers, do a pathetic job in preventing fare evasion. And when they do bother stopping you (for not showing a photocard that I have kept hidden for the last 25 years) they are rather rude.