If you weren't born smart, then you'll have to work at it. Assuming there's nothing seriously wrong with you, like amyotrophic lateral sclerosis, then there's only five things you really need to do:
My article on nootropics will do you no good if you're physically unfit. It isn't necessary to have a model's physique, but it is necessary to have a heart that can really pump blood and lungs that can really breathe air.
Research on both man and mouse has uncovered an improvement in cognitive function after aerobic exercise, from running to brisk walking. Weight lifting, while important in a fitness regimen, did not lead to a measurable, sustained cognitive boost the way that running did.
The fatty acids in fish oil (called the "omegas") are used to construct the myelin sheath of each brain cell. There are other sources for it, which you should find and add to your diet, but fish is a habit you want to keep. I personally supplement with a "Triple Omega" from Nature Made.
The vegetables are for the vitamins, iodine and fiber. The vitamins and iodine are critical building blocks for the brain, too, and the fiber makes sure all that crap going through your digestive system gets flushed out properly instead of hanging around and poisoning you.
It's funny how an article that aims to make you smarter first tries to make you healthier, but it's about as funny as curing cancer by first getting the patient to quit smoking. Artists are taught that every change in a model's pose affects every other part of the body--you move a leg, and the flesh shifts across the waist and tugs differently on the arms. Your body and your mind are not so separate either, they're not house and occupant, a change somewhere in your body means a change somewhere in your mind.
Just the act itself matters, and not just what you read. Although it can likely benefit you materially if you read about the things that concern your profession, and it'll likely benefit you cognitively if you read things that challenge you: logically, mathematically, philosophically, politically challenging. You'll benefit just from the act of reading. But here you are, reading already, so now it's time to look for books that'll tell you something you didn't already know. Look for books that might tell you something that'll still matter even if your profession is made redundant by machines someday, or a shift in the economy.
Subscribe to Nature, you can afford it even if you think you can't. Then try to understand the articles. Go to Wikipedia or a search engine every time you read a term or a word you don't understand. Struggle, it'll be hard. And then one day you'll know what a transcription factor is, or adenosine triphosphate, and your IQ will have gone up not because you've now memorized something, but because you've exercised learning and comprehension mechanisms in your brain and they're getting better at it.
Teaching is the other side of reading, because it's the QA--or "Quality Assurance"--phase. Teaching as a job is not what I mean, though. This would be teaching accidentally, or incidentally. Explain how something works to someone. Get your drinking buddy and explain the carbon cycle to them. And if they ask you a question you don't know the answer for, or you find you can't explain it after all, then you didn't really know it, did you? Don't be embarrassed, just go "aw, crap, I guess I gotta read more about that" and hit the books again.
Plan a lesson in your mind, and practice teaching to your shadow. Imagine someone asking "why?" at every point you were going to gloss over, and go back to the books until you can answer it.
"Pics, or it didn't happen" is what they say about grand claims. "Teach, or you don't know it" is the analogue.
And likewise you'll never really know--and never really think--until you act on it. Take the plunge sometime. Accept the possibility of screwing up and losing something and do it anyway. You must put yourself in the position you have to be in for your mind to do what evolution designed it to do: think fast and creatively to save your ass from trouble. Until you've been there, you have no idea what thinking really is, and you'll never be smart.