After crawling cites a bit from Wikipedia and other sources it seems like there are, at least, studies indicating successful improvements in working memory capacity from cognitive training. The DNB task is incredibly demanding on working memory, so it seems likely it is successful at least for improving that.
Furthermore it seems that some researchers suspect deep connections between working memory capacity and fluid intelligence, so there's still a plausible connection here--which makes the methodological issues even more frustrating.
Transhumanists have high hopes for enhancing human cognitive abilities in the future. But what realistic steps can we take to enhance them now? On the one hand Flynn effect suggests IQ (which is a major factor in human cognition) can be increased a lot with current technology, on the other hand review of existing drugs seems rather pessimistic - they seem to have minor positive effect on low performers, and very little effect on high performers, what means they're mostly of therapeutic not enhancing use.
So, fellow rationalists, how can we enhance our cognition now? Solid research especially welcome, but consistent anecdotal evidence is also welcome.