#1: What is it's purpose? For medical reasons, or as part of the oral tradition?
There are those who do not do metzitzah at all, there are those who only do not do it on Shabbos or Yom Tov but do it on weekdays, there are those who do it at every bris.
Of those who do it, there are ultimately two approaches: A. putting the mouth directly on the wound (commonly known as "metzitzah b'feh" (MBP in blogosphere)), and B. using a tube to draw out the blood with the power of the mouth, but without any direct oral contact from mohel to baby.
I wrote on article on the subject of sterility in a bris which addresses this subject.
But for those who advocate the MBP (direct oral contact approach), I think I found a source for you. (And yes, this is tongue in cheek)
On the verse Devarim 30:19, the Sifsei Kohen (Shakh) writes [referring to 30:6]:
[THAT WAS THE VERSE - HERE IS THE SHAKH:]