For more videos like this, click on Cleancare Videos.
Vacuum cleaners have huge price differences. Sometimes you don't get what you pay for, so be careful. A long with professional steam cleaning every 6 months to a year, regular vacuuming (as often as once per day if you have multiple kids and pets, or AT LEAST once per week) is the most important thing you can do in maintaining your carpet's appearance and prolonging it's life.
This video shows Consumer Reports' test criteria. I think they do a great job of replicating real carpet soil issues in their labratory setting. This video recommends their top choices, but to see their complete tests and recommendations, you'll have to join Consumer Reports.
My personal tips: If you are going bagless, look for a cannister with a drop release button at the top of the cannister, rather than down at the bottom by where all the debris comes out. That way, you can stick it down into the trash bag and actually close the bag around the cannister while emptying it. You won't have to breathe a cloud of dust and get it all over your hands. Also, that silly ball on some Dyson machine's drives me nuts. Who has trouble turning corners with actual wheels? Not me and in fact, I think the ball is more hard to manuever than wheels- don't pay extra for this marketing ploy!