What prayer do you recite when posting a mezuzah?
Believe it or not, the bracha for mezuza is right there in the relevant Wikipedia article:
Blessed are You, Lord our God, King of the Universe, Who sanctified us with His mitzvot, and commanded us to affix a mezuzah.
I'm not copying in the Hebrew text, since I'm less willing to put God's name in Hebrew all over the place than I am to do so with the English word "God." You can find it in Hebrew there or in most siddurim.