carpet cleaning

I've been in my house for almost 3 years and I still haven't done a full shampoong of the carpets. They were professionally done by the last owners as part of the sale agreement, because they had 3 small boys and the carpet was pretty bad. They only paid for the cheapest cleaning, though, which just shoots the water in and sucks it right back out, no scrubbing. I have a little carpet cleaner that does the same thing and I have used it on the high traffic areas a couple times, but not done a proper shampoo. At this point, I probably won't shampoo this carpet, because we will be replacing it in the next year (hopefully).