This book helped me out early on to mix things, some good monitoring speakers or headphones are a must. I like my AKG K92 headphones, I got three of them now because they're so comfortable to listen for extended periods and have a good range. You can real low frequencies really good, like sub bass you usually can't in a lot of headphones (good for symphonic sounds). The only drawback is that I'm used to hearing a lot more bass, so at the moment you can hear it, it's good. Sometimes I turn them up loud to see, and they're really good at all volumes, so that's not really a minus, per say. Just more accurate than what I'm used to. They're really easy to match mixes of commercial recordings by ear, as I'm finding out more lately.

http://www.akg.com/pro/p/k92

And they're about $50-$60