Are you on a clay soil? you need to drain your lawn and level off the uneven surface. If you need any information on how to drain it come back.

Hi and welcome to GoY is the area damp all year round or wet in winter and dry in summer. We need this info in order to make recommendations.

once you sort out the damp i would terrace it like in 2 or three steps depending how steep the slope is.infact doing just this just mite sort out your problem.its hard to tell without a picture .

I've added this to GoYpedia Sloping Garden Ideas...
.. click on the heading over to the right of this page...
..might give you some design ideas, such as the terracing suggested by Noseypotter.
I hope this helps :o)

When a garden is designed its drainage is usually the first thing to be sorted out. Otherwise everything you do after that will have problems. I would suggest you put in several drainage pipe channels going to the lowest point and allow for a pond, water garden or soak away. A lovelly feature can then be made of your "misfortune". Many of the top gardens, even C.Browns have been fashioned in this way so you will be in good company. It would also help if you could find out where the water is coming from. You may have a burst supply pipe near by or an underground spring.