(this is a rough sketch based on my research, which involves reviewing cognition enhancement literature)

Improving cognitive abilities can be done in a variety of ways, from excercise to drugs to computer games to asking clever people. The core question one should always ask is: what is my bottleneck? Usually there are a few faculties or traits that limit us the most, and these are the ones that ought to be dealt with first. Getting a better memory is useless if the real problem is lack of attention or the wrong priorities.

Training working memory using suitable software is probably one of the most useful enhancers around right now - cheap, safe, effect on core competencies.

When it comes to enhancement drugs, my top recommendations are: 1) sugar, 2) caffeine, 3) modafinil (and then comes a long list of other enhancers). Sugar is useful because it is effective, safe, legal and has well understood side effects. Just identify your optimum level and find a way of maintaining it (this requires training one's self-monitoring skills, always useful). For all drugs, there is a degree of personal variability one has to understand. Caffeine is similar, and mainly useful for reducing tiredness symptoms rather than boosting anything. Modafinil has some useful stimulant effects, appears reasonably safe and in particular doesn't seem to impair considered choice like amphetamines does. Metylphenidate may have its uses, but it depends on dopamine levels. Nicotine is a reasonable memory enhancer, as long as it is taken in a healthy form (gum, lozenges etc). Not sure piracetam actually works, and ginkgo biloba appears to be mostly a vasodilator (= good if you have circulatory problems, but perhaps not otherwise).

Healthy lifestyle matters. As I remarked in another comment, exercise has documented effect. It is rational to do not just for health but for cognition (so why don't I exercise? I need an anti-acrasia enhancement more than IQ!) Getting enough good sleep also improves performance as well as memory consolidation. Health in general appears to promote better cognition.

Learning to control one's mind is useful. A lot of people allow themselves to be distracted, annoyed or mentally sloppy. Doing a bit of internal cognitive behavioral therapy to identify bad ideas and behaviors, and fixing them, is a good idea. Easier said than done, but virtue is a habit. Just ask yourself whether you would like to have any given behavior as a habit, and then act accordingly (the nanoversion of the Categorical Imperative). Memory arts and other mental techniques can be useful, but are usually too specialized to be generally intelligence enhancing (I use memory arts just to remember grocery lists and ideas I get in the shower). Relaxation techniques (or just an awareness of how one works during stress) are very useful in everyday life.

Finally, the key thing is to exercise the mind by giving it challenges. Trite, but true. We tend to develop our skills best when we are at the edge of our abilities, not when the situation is routine or well-controlled. Hence attempting to climb higher cognitive mountains is both healthy and useful. If you have mastered special relativity, go for general relativity - or try to understand what poststructuralism really is about. As Drexler suggested on his blog, acquiring a broad knowledge of what exists in other fields is also useful.

One interesting finding shows that one's beliefs about the improvability of oneself strongly correlates with actual performance increases when training. I do not know whether this is true for all cognitive domains, but I wouldn't be too surprised if it was generally true.