What kind of music is it? Your first job is to find a mastering engineer who knows the genre, and what it's supposed to sound like. Your second job is to explain to him, maybe with examples, what it's supposed to sound like.

He then either will try to make it sound like that, or tell you "I can't make it sound the way you want, you need to go back and fix this thing or that thing with the mix or arrangement." But it starts with having someone who knows the sound you want.

There are a lot of great mastering engineers out there, but if you have a hip hop album and you send it to someone who does acoustical music, or vice versa, you won't be happy because they won't know what it's supposed to be like.
--scott