Assalam Alekum Respected Brother.
I dont know whats this "Sin no #1 thing neither i wanna know,but i can say this for sure this is not "Shirk".Its simply a thing that help you divert your attention.Its like a brain excercise,cant be taken as shirk as you seems to be very logical and trying to make your self pure just for the sake of Allah SWT.
But did you ever think how long will this exercise stop you from that sin??Wanna know a better excercise?Its obviously,"Salat".
"(29:45) (O Prophet), recite the Book that has been revealed to you and establish Prayer. Surely Prayer forbids indecency and evil. And Allah's remembrance is of even greater merit. Allah knows all that you do.
If you offer Salat and still it does'nt help,you should keep the company of religious friends and people.Reading good books change your mind some how but to practice good deeds,we need good people around us.Its like a motivation.
Also,i found this page,they have some really good tips that will help you Inshallah more than the Rubber band do.
May He SWT help you following the right path,keep all goods near to you and keep all bad deeds away from you and make the right path easy for you to follow.Amen.