What I do when one something inappropriate pops into my mind uninvited is say "rachmana nigar bei b'soton - may the Merciful One battle the Yetzer Hara". This very powerful segula was given by the Yetzer Hara himself to one of the sages of the Gemorah. It always works, guaranteed!
My father once told me that if an inappropriate thought comes to mind during tefillah, a person should press his big toe against the ground... it really works!