Apart from keeping them off the grass all together your only real solution is to hose down the area (as soon as possible after they have done their business) with huge amounts of water to try and dilute it. Would it be possible to build a run of some kind to keep them to a certain area?

its the only true way as ian sais . a dog run .you could walk them lots every time they eat or drink but thats not realy viable . i have 4 dogs and a dog run . my dogs dont suffer and niether does my garden.

My dog ruins our lawn too but its only a smallish one so dosen't impact too much.
In winter we fence it off as thats when it suffers the most and he is confined to the patio areas.
Next year we are probably going to invest in some artificial grass, we don't have a huge lawn area so although pricey it won't break the bank. There are some very natural looking artificial grasses available which are often advertised as pet friendly as easily washed down.

sounds like a good idea though poo is a bit of a pain to pick out of it is all.

Thanks to all yes I have connected hose and periodically hose the grass down/
It is still a bit patchy but hasnt got any worse! I

good glad of that