I put a piece of Velcro on my dashport and the other piece on the smart tag.
Every morning when I go to work I take the smart tag from my side compartment and the Velcro holds it in place.
As soon as I clear the toll I just remove and put it back to my side compartment.
If I don't do this sometimes I forget and this results in some angry drivers behind me during rush hours.
I also don't hang the smart tag holder on the windscreen because it is just telling everyone that I do have a smart tag in my car.