Save tools like compression for the mastering stage, instead of using them on the mixed project. Start the mastering process with limited amounts of compression, EQ and limiting. Compressors narrow the gap between the loudest and quietest part of the song. Overuse of compression robs the track of valuable dynamics. Apply EQ in moderation, using it to increase specific frequencies of different instruments. For instance, you may want to bump the low end below 250 Hz if the bass or kick drum sound buried. Small amounts of EQ on the spectrum between 1 kHz and 3 kHz can help make vocals sparkle. A limiter is used to limit the peak loudness level of the track, and should be used sparingly, mostly if your track has one or two sharp peaks with the rest of the track much quieter.
Listen to the first master of your song in multiple environments. Try listening on a home stereo and in car, as well as on a computer. Take notes on what you hear. Ideally, you should have a mix that sounds good across all environments. If the first master doesn't sound good in one or more settings, you will need to go back and use more compression, or EQ, to get it right. This process may take several listening sessions and experimenting with different settings before you get it right.
Part of preparing the CD involves sequencing the tracks and getting the album to flow. The tracks should not have wildly different volume levels, such that the listener feels she needs to turn the volume way up or down on each song; this is part of what using compression is for. When sequencing the CD, you want to start with a strong song that is ideally representative of what you as an artist are trying to accomplish on the project. The first song doesn't necessarily have to be the strongest; in fact, it's often better if what you feel is the best song is somewhere in the middle. Just like the music, the CD should peaks and valleys spread out across the length of the disk.