It's been eight years since we built this house and we still have the original carpeting. We've thought of having it re-carpeted but we had other expenses. So we opted for the next best thing, steam cleaning the carpets. We didn't have to buy one, instead we borrowed a steam cleaner from my in-laws. It did a great job cleaning the carpets, it looks like new almost. The dirtiest was of course on the stairs. Mind you, we don't wear shoes upstairs but it still got dirty. There are a lot of steam cleaners you can buy and use for your home, like the Ladybug vapor steam cleaner. If you don't want to purchase onem, then you can rent one from the grocery, I see those almost anywhere.
Along with steam cleaning, you should always vacuum your carpet regularly, of course. If it gets stained, always use a stain remover and not let the stain sit and dry out, then it'll be much harder to get rid of.