Lower the volumes of the tracks that are causing the bass issues. Bass guitar, kick drum and keyboard sounds are usually the three main tracks or groups of tracks that cause a muddy sound overall and issues with too much low end. Simply lower the volume of the tracks, while making sure the tracks are still heard, which may be the only adjustment you will need to fix the problem. Lower the bassiest sound first, but make sure you can still hear the track clearly and follow with adjustments to other tracks with bass-heavy sounds.
Use the on-board equalizer on some of the bassier tracks. This way, you can clean up the low end by cutting frequencies around 200 Hz. This is the frequency that can cause a bass track to sound muddy and very loud, although it is not technically loud. You can also increase the treble of some of the bass-heavy tracks to make bass instruments sound clearer and less muddy.
Compress the tracks that contain large amounts of low end that seem too loud. Often, simply lowering tracks buries them in the mix and they are hard to hear, but by adding compression to just the low end, you can clean them up without effecting their volume. Many multi-band compressors will allow you to only compress certain frequencies, so add a compressor and have it kick in at a low threshold but only compress the signal and frequencies below 300 Hz.
Using a high-pass filter is a quick way to help solve any bass issues that you have after mixing. The high-pass filter only allows the frequencies above a certain frequency pass through and be heard. Many muddy recordings can be cleaned up using a high-pass filter set at 100 Hz. Because most bass guitar tracks and keyboards do not have filters put on them, you may need to add them to other tracks to eliminate unwanted bass that is causing excess loudness in lower frequencies. Vocals and guitars are muddy below 100 Hz, so adding a high-pass filter to these tracks can help solve these issues.