Note all of the areas of your lawn that your mower cannot get to. This probably includes around your garden and other landscaping features, along walkways and beside your driveway. Up against your house might also be included.
Cut down long lengths of grass that seem to be growing sideways using edging shears. These should be used at least once per week.
Trim down the grass that is growing over edges, such as over the edge of your driveway, with a half moon edging spade. This will give a clean line between areas.
Use a standard string trimmer to cut down grass that's growing around a tree trunk, along a garden or beside bushes.