At this point, you've completed the tracking stages and are ready to finalize the project through mixing and mastering at home or in a professional mastering house. It's time to turn your demo into a reality.
If you're working alone, mix to your heart's content. However, it's always a good idea to get some fresh ears on your work every once in a while. Opinions do count here. If you're working in a band situation, don't mix alone, even if you have the equipment. It's a band project and everyone's opinions count. Listening to others ensures that you get everyone's input and help. The more ears on a mix, the better it will most likely come out. Once you've decided on a final mix candidate, burn out a few copies and listen to the work on as many different systems as possible. Try cars, home stereos, computers, iPods—you name it. If it sounds good and clean, you might be able to put it out for the world to hear. If it doesn't, go back to the studio to even out the rough parts.
When mixing for extended periods of time, it's important to pace yourself and take breaks. Ear fatigue can make mixing very difficult. For every hour you mix, take a fifteen-minute break. Get up, walk around, grab a bite to eat, and let your mind wander.
In recording, mastering is the last stage in album production. For a demo, however, you might not need it. Mastering generally deals with loudness, balance, and song sequence. If you've done a good job with levels during your recording, you might not need any loudness maximizing to make your signal strong enough. Depending on the purpose of the demo, mastering might not be worth the extra money. If the demo is not destined for release and is solely for solicitation of work and record contracts, mastering might not be necessary. If you feel that you need it, try mastering at home. There are many mastering plug-ins available for the computer-recording world.