When your mowing your lawn its important to make sure you keep a sharp set of blades on the mower.  Obviously the amount of grass you are cutting will determine how sharp you blades are but there are several other factors to consider.  If you are hitting rocks, sticks in the yard or other debris this will all affect how shape the blades stay.  Cutting tall thick grass or wet grass will also affect how dull they get.  When your blades are sharp they will actually cut the grass.  Dull blades tend to pull and tear the grass.  A easy way to see this it by looking at the ends of the grass.  If they have a frayed look then it may be time to sharpen them.  If like most people you are mulching your grass it is even more important to have a sharp set of blades to make sure the grass is getting chopped into fine pieces that can easily break down into a natural mulch for your grass.  If it is not it can lead to a thatch build up sooner than normal.  It is a good idea to change them at least once on month if possible.  This can be a easy task by simply using a angle grinder in your garage.