Meditation. Jason Miller said much the same thing in the Strategic Sorcery Course. I think meditation is the most crucial skill a magician can have. It's something I do as consistently as possible. Occasionally I miss days, and I can always tell when I miss a day because I'm less focused. Meditation is similar in benefit to exercising on a daily basis: It keeps you toned.
There are some magicians who will say they can't meditate, but I don't buy that. They can meditate, but they're unwilling to discipline themselves enough to do so and they've likely bought into the popular idea that meditation is just about emptying your mind (that's one form of meditation, but not the only kind). The key is to accept that if you do have random thoughts arise, you don't give up. You note them, maybe even follow them, and then return to the breathing, letting it carry you deeper and deeper into an altered state of consciousness. It takes some work, but if you are consistent about it, you'll succeed eventually.
I think one reason people say they can't meditate is that they are so distracted by every thought and emotion, and perhaps even afraid to face those thoughts and emotions. Meditation does give you the chance to work through thoughts and emotions and sometimes its not fun. But doing it frees you of those thoughts and emotions. It gives you clarity, focus, awareness, and perspective.
Meditation teaches you to sort through the chaos of your internal reality. It's doing the internal work, so that the external work of living your life and manifesting your magic is a lot easier. Certainly in taking up a dedicated practice of meditation I have needed to do less overt acts of magic, save on the rare occasions where its really warranted. By doing the internal work, my life has gotten much easier. It didn't happen over night, buts it a cumulative effect if you are consistent.
So that's the one technique or practice I'd keep. What about you?