My lawns are old paddock grass. Full of couch grass. I keep the lawn mowed regularly and the edges of the lawn are trimmed also on a regular basis. This way the couch is contained and actualy behaves quite well. infact it merges in with the other blades and is not visible as couch. Maybe you have clumps of thicker bladed grass that lays in a circle looking unsightly or sprawling across the lawn. some forms of ryegrass can look similar to couch. Look up on the net the different forms of grasses and how to treat them. Sorry I can't be more specific, but without seeing the actual plant it would be difficult to advise on action.

I think 2ndhand's advice is pretty sound. The history of gardening is often the story of gardener against couch grass. Unless you are willing to blitz the area with a very strong herbicide (one that kills everything) and then start again from scratch a good year later then it is darned nigh impossible to eradicate. A horticulturalist friend of mine who helps me with the garden tells me that her son in Australia has managed to find a selective weedkiller for couch grass but the bad news is that the active ingredient is not legal to sell in the UK.

As others haver said unless you're prepared to kill all the grass and start again you are stuck with counch grass in your lawn.

If there isn't too much of it you could let it grow and then paint weed killer onto it( keeping it off the other grass)

Other than that at least once a week mowing will keep the grass looking passable.

Supposedly close mowing is supposed to get rid of it, but it ain't worked here. You could always let it grow and dig up the roots for food (eaten in Italy I am told).