Tip #1 Belt
The belt on your vacuum cleaner needs to be changed at least every six months, (unless you have a lifetime gauranteed belt). Belts stretch out over time and loose their ability to clean efficiently. The belts spin the brushroll which disturbs the dirt, allowing the air to move the dirt out of your carpet.
Tip #2 Brushroll
Your brushroll needs to be able to spin freely. Hair gets caught in the bearings and bushings (the ends of the brushroll) and prevents the brushroll from spinning, which prevents the vacuum from cleaning as well as it should.
Tip #3 Bags & Filters
Your bags and filters need to be replaced often - vacuuming is more about air flow than about suction and when the bags and/or filters are too full it prevents the air from moving the dirt properly.