The following are mistakes I've seen myself make...
1. Don't fall in love with your sounds. Often you'll quickly create a loop, throw a kick drum on there, add some chords, add a bassline... and then you spend the next 2 weeks listening to those sounds over and over while you try to finish the song. This makes those sounds sound "right" to you. But they may not be. Try changing the kick, try changing other parts. When you do it will instantly sound "wrong" because you've grown accustomed to your first choices. But give it a chance and see what really works.
Related, sometimes there's a sound that just doesn't work with the track. It still sounds "right" to you but it's muddying your mix. Don't be afraid to at least consider deleting any track of your project.
2. Don't add an effect to a track unless (a) you understand what it does, (b) you can hear what it does, and (c) it improves the track. It's really easy to get in the habit of adding a compressor to every track, and maybe a saturator, but be sure they're helping your mix, not just adding noise and mud.
3. Don't overdo layering. I used to find 4-5 patches that sounded good for my lead melody, then basically use all 5 of them at various volumes and EQs in the hopes of creating a nice fat sound. I've found layering works much better if you pick one patch that is "almost" what you want the final sound to be, then add 1-3 layers that are subtle but add something to the sound. A layer should be adding something specific, like low-end or high-end or a transient. And like rule #2, if you can't hear a clear improvement when you add a layer... get rid of it.
