Honestly, I'm going to say practice. None of this is paleo per se, it's just advice about how to focus. Sorry it's kind of long.
I used to teach grad-level standardized tests on the side, and I'd make my students (a) meditate for 30 seconds before each section; (b) create a ritual; (c) practice like their life depended on it or not do it at all.
The more you half-ass your way through anything, the more your brain becomes habituated to half-assing it, and that's a hard habit to break.
The main point of the meditation and exercise isn't what you'd think (after all, how much Zen do you really achieve in 30 seconds?) but rather as practice in focusing. In 30 seconds, you can get a very good sense of exactly how slowly you can make time pass when you're focusing (a big bonus when you're crunched for time). Understanding the passage of time and learning to temporarily ignore all outside stimuli is key.
I find rituals important, like doing 5 pushups or running around the block (plus the 30 second meditation) right before you launch into any tasks requiring extreme focus. It's physically stimulating, but it also functions like Pavlov's bell. If you do the same thing every time, your brain learns that this ritual means focus time, and so the lag between the ritual and maximum focus shortens with practice until it becomes second nature -- five pushups, 30 seconds with closed eyes, and suddenly the lights are brighter and you're in the zone.
So, I don't know if any of that is helpful or what kind of focus you need to achieve right now, but when I get a new student who despite being smart and together just can't focus, I really treat it like marathon training. Do it every day. Some of us are very used to being focused, for some of us it's a learned skill, but it can certainly be learned.
Last note: I know everyone's all anti "chronic cardio," and I don't love it to death either, but I do find it useful for when I really need to get back into focus mode, because it's boring as hell, which also makes it endurance-training for your brain. It helps train you to power through things that are unpleasant for longer than you want. When this is what I'm going for, I don't listen to music, I don't daydream, I just run and stare at the clock for 30 minutes. I suppose you could also do something else unpleasant for 30 minutes, but I feel like at least here you're getting some exercise (which, like everyone else said, helps make you smart anyway).