Mastering Tips

Get the Ozone mastering guide its free. http://www.izotope.com/

This is a direct link to the Ozone guide John mentions.
http://downloads.izotope.com/guides/iZotopeMasteringGuide_MasteringWithOzone.pdf
 
+1 for Roberto's comment on the book by Bob Katz.
http://www.amazon.com/Mastering-Audio-The-Art-Science/dp/0240808371
 
I have read this book and often refer back to it. I will say that sometimes he talks for moments about things it took me days of web research to understand. Consider Mid-Side Processing (the traditional 2 Mic Recording method for smooth mono and dynamic stereo).
 
I think Bob Katz's book is useful if you are really serious, but I will say that a few years back it would have meant significantly less to me than it does now. I would have glossed over lots of important items saying to myself, oh, I know that.
 
Another example is his discussion of how to use a vector scope to quickly identify phasing issues. I've been playing with a vector scope for a while, in conjunction with using Melda Productions M Auto Align.
 It's tricky to understand. I know where to look for phase issues on the vector scope and I know it can be used to help, but I'm still learning how to apply it to guide me to fix things I'm not succeeding in hearing. Or using these tools to become a better listener. Honestly, watching tv and movies along with use of the vector scope has helped a lot. I find TV shifts back and forth from mono to stereo often. Also the Beatles. Those guys made wild mixes for then and now.

I say go sit in on a session at least once. Take just one song you'd like to have Mastered. Nearly every (good) Mastering house invites clients in and can do a single song for about $35-$75 depending on how in demand the engineer is. Take a seat with the engineer, and discuss how you want your song to sound. Ask what tools they use and what they like. You can learn sooo much from just a single session. If you need to travel to a Mastering engineer, there's at least one decent candidate within every Major U.S. city, or in Europe, etc. However, if you're within 200-300 miles of Nashville, L.A., NYC, or even Atlanta, Miami, Seattle, or Chicago, it's worth the extra drive and a hotel room, believe me! But don't go to a studio that "also" does Mastering. Go to a studio that only does Mastering.
 
Some Mastering engineers don't like to share their money-making secrets, while others believe they don't have anything to worry about because they already have a good customer base. In either case, you can quickly get him to tell if you ask what he despises with other engineers or mixes. Get him to tell you what separates him from the rest. Show interest in the tools he uses and be impressed. Let him show off for you (not in a testosterone-ish sense).
 
Now, the very best engineers will love this interest. Bad engineers will shy away from discussing anything with you, because they tend to feel they're totally ripping you off because they believe it's all so easy. They're also so afraid of competition, when in fact, the BEST thing they can do is educate you. The more they are willing to work with you, the more likely you are to seek them out in the future!
 
Then, pick up Bob Katz' book and enjoy the techie stuff!