I had started compiling a small list of ways to avoid/attack otherwise fatiguing sounding mixes, and I realized that I basically was just describing the basics of tracking, mixing and mastering in general.
Things like mix up your mic and preamp selection, follow a quality gain structure and moderate recording levels, sweep the tracks for resonances, use arrangement/panning/EQ/depth/automation etc. to give each instrument it's space, mix for sound quality and not for volume, be careful of too much buildup in the upper mids, and decide on final mastering volume based upon your ears and not some set of numbers.
And the big one; if something sounds fatiguing, fix it; if you can't fix it, get rid of it. Don't be afraid to throw out what doesn't work.
G.