From PCRecording.com (though the advice applies equally to recording on a Mac,) here are some basic things to consider when mixing:
One major mistake many make is to mix by addition rather than subtraction. That is, if you cannot hear one track well enough you turn it up. In addition, the level changes are oftentimes in increments that are too large. Take it easy, make small changes. Try bringing down some levels rather than just boosting them. Even changes as small as .1dB are significant and can be easily discerned.
The article also touches on hardware latency and software buffers, which are often overlooked as part of the mixing process, but can be a source of frustration for beginners and experts alike.