(1) The greatest antagonist to consciousness/intelligence is fear, that is, the biochemical emotion/experience we call fear, the fear/flight side of the fight/flight adrenaline neurohormone.

(2) Pretty much all modern humans live in a state of low-grade emergency, characterized by a chronic overproduction of adrenaline, chronic neuromuscular hypertension, and chronic imbalances in sympathetic and parasympathetic neural systems. 

This is the place to start. Practice relaxing.

Most breathing exercises are essentially relaxation exercises. Most mantra exercises are also primarily relaxation exercises. (The mantras content, it's words, are orientation-effecting, but it's repetition gives a certain area of the mind a "busywork" that quickly lulls the whole mindbodysystem of the practitioner into a very useful and healing relaxation.) All isolations-- that is, exercises in which an important element is that the practitioner be alone, or in some special place-- are relaxation exercises; because the foremost cause of our chronic low-grade emergency problem is crowding and overexposure to stranger-therefore-potentially-hostile humans. And so on.

Here's a bit of bait for you. Relaxation is the key to the siddhis, that is, to those so-called "Psychic Powers". Seriously. You want the powers? You've gotta learn to relax, make relaxation, neuromuscular awareness and self-control, your own personal fine art.