Don't do drugs or drink when mixing. Some alter your perception and some alter the physical response of your ears. Definately stay away from coffee , your mixes will sound bright and thin. Infact the only stimulant I have found that seems to have no side effects and helps long hours in the studio is Ginseng.
Also take regular breaks to shift position and keep aware of your breathing. Lack of oxygen can have suprising physiological effects ( no sh*t sherlock ).