Start by paying very close attention to each recorded track individually. When you have all tracks playing in a mix, it can be difficult to identify troublesome sounds that are negatively affecting the mix. Listening to each track in isolation first will make any errant clicks, pops and hisses very obvious, enabling you to edit them accordingly before you begin mixing.
Listening to tracks in isolation may work well for identifying shortcomings in audio quality, but it can only take you so far. If you're working on shaping sounds with EQ and effects on tracks separately, there's every chance that these tracks may sound rich and full in isolation, but will only serve to muddy the mix when all of the faders are up. Adding EQ and effects takes up valuable headroom and can result in a messy and overblown mix. So make sure that when you're fine-tuning a part, you're doing it in context of the piece of music and making it work with the other sounds in the mix.
Subtle changes in EQ of a part can work wonders with its apparent, or perceived, loudness without having to boost its level. Sometimes a part is competing with another instrument in a similar frequency, and rather than simply boosting the volume until it sounds loud enough, it's wise to try and carve out a place for it in the mix by cutting and boosting different frequencies in the competing parts. Often, that's enough to give parts their own sonic space so they can be heard more clearly.
Sometimes, different tracks compete with each other to be heard clearly. Simply raising the volume of each track busies the mix unnecessarily. It's possible that a little experimentation with the pan controls can solve the problem by moving the track's placement in the stereo field, allowing it to escape from the shadow of another instrument that was panned in a similar place. In the case of two electric guitar parts, for example, panning one left to around the 9 o'clock position with the other panned right at around 3 o'clock, will give the parts plenty of separation, enabling each part to be heard more clearly, as well as enhancing the stereo image.
With computer-based recording and editing, it's easy to get caught up in the clinical perfection that mixing with a mouse can bring. One thing that is missing from the days of analog mixing consoles is the ability to add a little more human touch to the mixing process by riding the faders as you mix in real time. To restore some of this human touch to your mix sessions, consider adding a hardware controller to your computer recording rig. This will allow you to manually ride the faders and bring more dynamics to your mixes
When played solo, your stereo keyboard or guitar part may sound full and impressive, but when added to other sounds, it can often be difficult to find a place for it in the mix without causing conflict with other parts trying to be heard. You'll then spend time trying to shape this stereo sound with excessive use of the EQ controls, all in order to make it blend with the other instruments. The fact is, that if it were a mono sound source, it would be far easier to find the right space for the part. All you'd likely have to do is tweak the pan control a little until it finds the right space to slip into.
To avoid ear fatigue and the bad audio decisions it often leads to, take frequent breaks when mixing. Mixing at high volume is another way to desensitize your ears, so try to monitor at reasonable volume levels. Also, for a little perspective, take the time to use reference CDs in your collection, and play them on your monitoring system so you can make quick comparisons to commercially released product.
It's easy to get carried away with the use of effects devices like reverbs, delays, flangers and compressors. When we first hear their effect on a dry sound source, it can be quite seductive, and it's easy to go overboard with their use, and then encounter negative results. Reverb, for example, has the tendency to move sounds further back in the mix, having a detrimental effect on the immediacy or presence of a track. Compensating for this by bringing up the level only serves to use up more headroom and accentuate the unnatural, spacey sound created by the effects device. A good rule is to set the effects level at the point where it sounds good to you, and then roll it back a little. A heavy-handed reverb setting on your vocal will sound worse after repeated listens, so it's best to use it sparingly so that you have results you can live with.