Hi
It's usually caused by poor drainage and/or lack of nitrogen. So best approach is to improve the drainage - scarify to remove any moss and then spike to aerate (you can add a mix of compost/sand into the spiked holes but simply spiking may be enough). You can hire motorised aerators of you have a large lawn but a garden fork will do the trick.
A feed high in nitrogen will help but NOT now as it is the wrong time of year and could cause more problems later on as it will cause lots of soft growth which will be vulnerable over winter. You can get autumn feeds for lawns which are low in nitrogen and applying this will probably help until you can apply the nitrogen richer spring feeds.
I think there is a spray you can get (RHS site) can probably advise but improving the general health of the lawn will often do the trick and be longer lasting.
Oh and don't compost your lawn cuttings as that could then reinfect your lawn.
Hope this helps 
