Mastering is best left to the professionals who specialize in it.
Here's the one thing everyone forgets......and one you should make sure never leaves you.
If it has been mixed correctly then there is no need for someone to come in and FIX it!!!
The mastering engineer just makes sure my low end isn't running away and my high end is not piercing. He makes sure I'm within redbook cd spec which is -0.3 db. And if i'm going to vinyl he makes sure that low end isn't rumbling so much as to overtly expand the groove into the next groove when lathing.
Thats it. The end. I'm always suspect of someone who wants to master their own stuff or their friends do it.
LEARN HOW TO PROPERLY MIX A RECORD FIRST!!
I can't emphasize this enough...and mastering isn't an issue.
Now...don't get me wrong...there has been the odd case/rare event of a half deaf mastering engineer doing our record and making it sound like crap when all he had to do was leave it alone. Haven't had an issue since we switched though.
"you bust your ass to make all the money you can...thinking it solves shit...and then you realize..when and if you get there at all...you got more problems and you missed out on the one thing you can't buy or get back...your life! Spend time with friends and family...You can be broke in other ways too!...."