Always mow with a sharp blade - A rotary mower cuts by blunt force. A sharp blade will cause the least trauma to the leaf and give the best results. A dull blade leaves wounds that are an entrance point for disease and give a dull white appearance to the lawn after mowing. You should sharpen your blades at least two or three times a season.
Return the clippings - Grass clippings break down quickly and are recycled back into nutrients to feed your lawn. You should only bag the clippings when too much time has passed between mowing and clumping occurs.
Get outdoors and enjoy.