Ornamental grasses require little maintenance, but they eventually need rejuvenation. If your ornamental grass has a large dead space in the center or the grass has been flopping for the last few years, it is probably time to divide it. You may also want to reevaluate the site your plant is growing in. If the flower plumes (called inflorescences) are sporadic or nonexistent, the plant probably does not receive enough light and, in addition to being divided, could benefit from being moved altogether.
Cut back last year's foliage before dividing. For small grasses, such as blue fescue (Festuca), use garden scissors or pruners to cut the grass back to a few inches. With larger grasses, tie rope around them to hold them in place. Large varieties, especially Miscanthus varieties, require stronger pruning tools, from hedge clippers to a weed trimmer or even a chainsaw. Cut larger varieties back to 6 to 8 inches.
Lift smaller grasses with a trowel or shovel, digging sufficiently under the clump so that you do not damage its roots. Larger grasses usually require more muscle. Well-established grasses may require, as the Proven Winners website attests, "a strong back, or three," to lift plants out of the ground. Dig down under roots. Use a crowbar, if necessary, to get the clump out of the ground.
After the clump is lifted, divide it into sections with a good chunk of roots in each. Use a sharp knife for smaller grasses. Larger grasses may be divided with a shovel or with the spade fork method, a dividing technique Tracy DiSabato-Aust describes in her book "The Well-Tended Perennial Garden" (Timber Press; 1998): Hold two spade forks back to back with the forks interlocked. With a single motion, move the handles in opposite directions and split the clump apart. If more muscle is needed, Proven Winners suggests an axe, a concrete saw or even a chainsaw as a last resort.
Take out any dead sections from the divisions. For smaller grasses, further division can often be made by gently loosening the roots, as long as there is enough of a root system for each planting.
Dig holes for divisions in chosen areas. Put planting mix into the holes, along with existing soil. Water the soil, and then plant your divisions to the depth they were in the original clump. Backfill the hole, tamping down lightly as you fill. Water the plant.