Tom Chivers has a
post on the Streatham & Brixton Chess Club blog that usefully restates advice given by such authors as Mark Dvoretsky and Jonathan Rowson (as well as, on a humbler level, yours truly): the best training is game simulation (e.g. training games and analysis exercises). It's not online blitz, or even, says Chivers, solving problems online or from a book. Rather, there's value in using genuine physical equipment in one's training, if only because that's how the real games are played. More info at the link above, along with some positions he recommends for training purposes.
HT: Brian Karen
The problem, of course, is that it can be a huge pain in the neck, especially when you're looking at a lot of subvariations in a book with very few diagrams...