I have noticed that many providers specify that clients request Greek in advance so that they have adequate time to "prepare".  What exactly does this preparation involve?  Silly question, but I'm a newbie and I'm curious.

They will need to "clean" their system so the experience is enjoyable and hygienic, reason why they require notice.

As Paloma said, prep usually means "cleaning out," which usually involves some fasting an enemas.

For some girls that may be as little as avoiding curry and taking a dump in advance.  For most it means being very careful about what she eats and drinks , and then doing an enema.

I provide anal on my clients meaning I bend them over, so to avoid a mess (caca) I will provide an enema to clean their naughty hole before playtime.