My friend tried the dog rocks and said they didnt work
http://www.dogrocks.co.uk/
but obviously they have worked for some people.
there is another product called green um
http://www.dogrocks.co.uk/
You could also try tomato sauce which is supposed to neutralise the ammonia in the dogs urine
We used to keep taking out the brown patches and re seeding, but a never ending job in a large lawn. We opted to remove all the lawn and lay slabs but kept our lovely borders and trees. Much cleaner in the winter, as our lawn resembled a speed way track!!