The best thing I ever did was invest in quality monitors (powered Wharfedale monitors) and quality headphones (Sennheiser).

Also, patience helps a lot. Even when you think you have it "right", record that mix and then tweak a little more to see if there's a "little mo' betta" mix in there somewhere. I have gotten as far as v12 and then decided that v3 was the best of them. A little more reverb on the solo, a little less echo on the vocal.... whatever. It's your canvas to paint any color you like.