Make sure that you only work with high quality tracks. When you record or import your initial tracks, you must make sure that you have no clipping anywhere in the track. If your signal exceeds 0 db, you will have a track that does not sound good in the final mix. When you record a track, make sure that the red light never flashes on your mixer, audio interface or software so that you know you have a clean track.
Complete a rough mix using two basic tools. Use volume faders to adjust the volume of each track, and the pan feature to put each track in the part of the stereo sound spectrum that you like it in. These two tools alone can help you create a rough cut without using more complicated effects. Make sure that the right track is highlighted the right amount. For example, on a song that features vocals, make sure that the vocalist is cutting through the rest of the mix and is heard clearly, and in the right proportion to the other tracks.
Take breaks from a single track when you can, and reference other tracks, such as a recorded and mixed source that you work in the style of. Otherwise, you can start to lose reference on the track, and something that you think sounds great won't when you come back to it a day later. Also, don't listen to the music with the overall volume too high, or you can compensate in ways that will make the final mix sound poor. A common mistake is to end up with highs that are far too bright.
Apply compression where necessary to the tracks. Compression helps to reduce the peaks of a sound signal that leaps out and sounds too sharp. Compression needs to be used on most tracks, but a common mistake is to overdo it and use too much compression. For instance, without compression, a vocal track will often sound far too variable, but with too much compression it will sound flat and lifeless. Err on the side of caution and apply a small amount of compression to help stabilize each track.
Use EQ to gently tweak your tracks. Like compression, a common error is to overdo it when applying equalizers. A straight boost across the spectrum is a mistake, as is large boosts in the middle. Be gentle with your EQ and apply it sparingly to cut or boost frequencies to get the best sound possible from the track.