Set your vacuum to the right carpet height setting. This will ensure that you are getting all the embedded dirt out of the carpet as well as the dirt that is in the top layers. Generally, frieze carpet will require your vacuum to be set on regular or medium high.
Vacuum the carpet thoroughly. Run the vacuum over each spot in the carpet at least four times, with "back and forth" being one time. This will not only pull all the dirt out of the frieze carpet, but it will also give the vacuum a chance to grab dirt that has been agitated with the beater bar. If you only run the vacuum over an area once, then it will just stir up the dirt but not pick it all up. Generally, this will be sufficient to clean your frieze carpet. If it still appears matted or dirty, though, you can take some more extreme measures.
Gently run the carpet rake over the frieze carpet. Usually carpet rakes are used to comb shag rugs, but they will work if a frieze carpet is still matted after a good vacuuming. Rake the carpet in straight lines, and do not push down hard or rake too aggressively or you can pull the fibers right out of the carpet. Instead, use the rake to texturize the carpet, and if it snags untangle it rather than just yanking on it.
Vacuum the carpet again. The carpet rake will have loosened up the fibers and additional dust and dirt. As before, run the vacuum over each spot four times. You may want to raise the carpet height setting slightly if you have raked the carpet up off the floor significantly.
Have your frieze carpet professionally steam cleaned once a year. Frieze carpets generally remain fairly clean with only home care, but once a year having your carpet steam cleaned will keep it from getting too soiled to respond to your regular vacuuming efforts and will prevent recurrent stains and unpleasant smells.