Turn down the monitors and listen.

When doing a mixdown from a recording where there are many parts that compete with one another, such as multiple guitar and keyboard parts in the same frequency range, a good technique is to slowly turn down the monitoring level until you hear certain instruments disappear in the mix. It’s also a good idea to make a tape or CD and listen to it in your car while driving. The road noise will mask some of the instruments as well. As you do this consider what you can accomplish by boosting or cutting certain frequency ranges of these instruments to make more room for one another. Sometimes you can make tonal changes that might sound terrible if the instruments were heard by themselves, but really help them to sit with each other in the context of the mix. Quite often removing just a few dB here or there can free up a lot of space for other things to be heard more easily.