Mixing and mastering
Mastering is truly one of the black arts. It began with the person who did the final step of transferring what was on analog tape to the acetate or vinyl master disc that the subsequent vinyl albums were physically pressed from. It has evolved into what has become "the loudness wars" where the effort goes into trying to manufacture a recording that is as loud or louder than any other recording it's compared to. This worked to a point, but eventually, in many cases it's wiped out all the dynamic range of many modern recordings and produces distortion to boot. There are many factors that govern how loud a recording sounds, some involving processing of the unmastered recording and some that are psychoacoustic and involving the interaction of brain & ear. One of the most important factors in determining how loud a song sounds is its arrangement. A group of acoustic musicians playing a tastefully arranged jazz or classical standard well will always sound louder than almost anything it's compared to. You'll pretty quickly exhaust the possibilities in processing and the quest for the golden plug-in never ends.
A better solution is to go for the psychoacoustic solution and fix the problem as early in the process as possible. You need to begin with a good arrangement where the individual musicians make a serious effort to stay out of each other's way both tonally & rhythmically. For example, a frequent problem is bass content in a guitar sound taking up space where the bass would normally sit. It's better to grapple with the problem before recording than in the mastering process. Another common problem is parallel motion in the music, that is, two or more musicians playing parts that are rhythmically identical. This really clutters things up and is difficult if not impossible to clean up when mastering. Every musician really needs to pay close attention to what the others are doing. The notes that you don't play are probably more important than the notes that you do play. There are exceptions, however. A good bassist pays close attention to the drummer & builds his/her bass line around what the drummer is doing (and vice versa)to produce the synergistic relationship that's the basis of the "groove" of a song. And if you listen to any kind of classical music, there is parallel motion EVERYWHERE but it doesn't present a problem in that context for 2 reasons:
1) Classical musicians have done hours of practice with a metronome & can usually keep rock-solid time, thereby avoiding the timing glitches that produce the aforementioned "train wreck" phenomenon.
2) Classical musicians usually have a conductor with really good time to serve as a common reference for time and pay attention to him or her.
Then we come to the mix. This is a very delicate balancing act because the machine is much less forgiving than the air. Mastering is pretty simple when there's a good arrangement & a good mix and even then, there is still a lot of robbing of Peter to pay Paul involved. If you're doing your own recording, keep all the tracks separate that you can and avoid track-bouncing if at all possible. That way, if you end up getting me to mix your tracks I'll have a lot more possibilities. And I don't consider myself a true mastering engineer. IMHO, I do a pretty good job but there are ethical people out there who do excellent work and I'd be more than happy to send you their way. A good one will usually be willing to do one track gratis so that you can make your own decision.
A good website that will give you some very good information in this area is: http://www.turnmeup.org/