How do I brew a sweeter beer?

To increase sweetness without nullifying the yeast you will need to add non-fermentable sugars to the wort. To add sweetness and some body in an extract brew, lactose is the usual adjunct: 1/8 to 3/8 pound in a 5 gallon batch. Lactose is non-fermentable and can be used to increase the sweetness either by adding at boil time or at bottling time.