Regardless of wether your PC lives in a room with carpet or timber floors, Cleaning the floors stirs up all the dust matter. Dont just drape a towel or something over your PC, that is bad. If you have extra cooling in your case, it is wise to have some sort of switching to be able to control the fans. If your PC is sitting idle or not doing any real intensive tasks, simply turn the fans off or slow them down. This can be a difficult thing to achieve but not impossible.
All cases have gaps and holes and venting and are all open doors for dust bunnies to enter.
Sealing up these extra gaps and holes as best as possible will stop them clogging up with dust and will force airflow to travel through the fans where it is supposed to. Once the airflow is directed through the fans as best as possible, the best way to reduce the collection of dust inside is to maintain a balanced or slightly positive airflow pressure. That is, have as much air coming out, as you have going in. There are a lot of variables involved like position of fans, amount of room in the case etc, but the fundamentals are the same. If you have 4 fans of the same size with one sucking air into the case and 3 blowing air out, you are going to have negative airflow pressure.