Sending {{{prayers and vibes}}} that all goes well for you. I've been told that nowadays the procedure is not nearly as uncomfortable as in the past; still, I wouldn't want one anyway. You might try a Lamaze technique and tell your self that you are getting an LP (lumbar puncture) instead of thinking of it as a spinal tap. During childbirth, I told myself that they were contractions instead of labor pains, and it really helped (I was taught that if a person hears "shot" they show signs of distress, while the word "innoculation" didn't get much of a reaction).
I hope that some of the others here who have had the procedure will let us know what it is like.
Please keep us posted on how it goes! And remember, it's okay to be nervous - and the staff has dealt with this all before, so let them know about your trepidations