When you call someone and ask them to come over and help you clean your carpet, you should know that you have already taken the crucial first step required to enable you to make the most of your rug in every way, shape or form that matters to you. A big part of the reason why that is the case has to do with the fact that the burden will no longer be placed squarely upon the two shoulders that are within your ownership, but rather it will be shared by trained experts who have spent years figuring out how carpets are ideally meant to be cleaned at any given point in time.
Suffice it to say that prepping for the arrival of your chosen carpet cleaning service is something that can be fruitful for you in more ways than one, starting off by making it so that they would have an easier time giving you a cleaner end result. One thing that you can do before carpet cleaning arrives is to spread some baking powder all across the surface of your rug, since that is something that can create a granular layer on the carpet which can rub against hardened dirt and break it into smaller and much more easily manageable pieces.
Now, there is no rule saying that you have to do this, and many carpet cleaners would be delighted to handle the prep work for you as well along with everything else that you have hired them to do. However, taking the initiative can make life easier for everyone, so you should consider doing it just so that your carpet gets that much cleaner.