The most important thing you can do to keep your carpet looking great is to vacuum it. Do this thoroughly and regularly, especially in high traffic areas.
For best results, use a vacuum with a beater bar or rotating brush and plenty of suction. Change bags and filters routinely, and keep recovery tanks clean. Studies show that even a well maintained vacuum can lose 80% of its "pick up ability" when the bag is 50% full.
Set adjustments according to recommendations for your type of carpet. Heavy traffic areas like entryways should be vacuumed regularly, daily if possible. Walk off mats at entrances can dramatically reduce tracked dirt. Less heavily used areas can be vacuumed less often, perhaps weekly. Vacuum three passes for light cleaning and up to seven passes where there's been heavy soiling or traffic.
Why is vacuuming so important? That's because most of the "dirt" found in our carpets is tracked in by us and our pets. This insoluble "dry particle soil" consisting of mostly sand, clay, and quartz is easily removed through regular vacuuming. If this dry soil is left to build up underfoot, the grit constantly rubs against your carpet fibers like sandpaper.
First it removes the stain resistant coating on the fibers. Next, it permanently scratches the fibers smooth surfaces, dulling the carpet's color. At this point, you can clean the carpet, but the beauty of your carpet in those areas in damaged beyond repair.
That's why it's also a good idea to periodically rearrange your furniture. By changing the traffic patterns, you can even out the wear on your carpet.
There's another compelling reason for regular vacuuming. Carpet is beautiful and beneficial, but it also collects pollutants very efficiently. This unhealthy material enters our homes in the air or clings to clothing, fur or feet. It settles into our carpets and onto upholstered surfaces just like a furnace filter collects dust and airborne particles. Once saturated, our furnishings start "spilling" contaminants back into our indoor environment. The fact that our homes today are tightly closed, highly insulated environments adds to the problem. We end up thoroughly exposed to unhealthy contaminants.
Industrial-type materials deposited in carpets include motor oil, acids, carbon, asphalt residue, lawn fertilizer, industrial gas residues, insecticides, herbicides, and fungicides.
In addition, there are also organic materials that can affect those of us who suffer from allergies. Organic deposits found in our carpets can include food crumbs, beverage spills, dead skin cells, hair, dead insects, insect droppings, residues from heating and cooling, pet soil, and other "biogenic" wastes. This biological debris provides a breeding ground for microorganisms like bacteria, viruses, fungi, dust mites, mold, etc.... Regular cleaning reduces and controls biocontaminants.