Weeds are the bain of a gardeners existence. Many people reach for something and spray. There are many cultural things that can be tried before spraying. Cutting the grass correctly and ensuring good drainage will go a long way towards preventing weeds. If water is puddling on top of the soil you know you're going to have to do something, some drainage tiles, bringing in more soil to level it out, somehow improve the drainage. With conditions like this you will not only have problems with weeds but funguses as well. Drainage is important. Cutting the grass higher can help and leaving the clippings on the grass shades out weeds. If you have shade you won't have a lot of weeds. If weeds don't germinate, if they don't have sun to germinate, weeds won't be as much a problem. If your lawn is healthy, has a good strong root system, if you have a nice thick lush lawn it will be hard for weeds seeds to get going in that environment.