Do puzzles, but not necessarily tacitical puzzles(tactics are a good source aswell). The ones like "White to move and win" or just "white to move" You have to find the best continuations and calculate the variations, and find the goal in the position.
How to train calculation skills
Sort:
I can highly recommend Chris Ward's book "It's your move: Tough puzzles". It sounds like exactly what you're looking for: for each problem, it presents five candidate moves, and you have to figure out which one you like best. The author then shows you the winning line, and explains what's wrong with each of the other candidates.
Some of these puzzles feature quick tactics, but most are positional and don't lead to a quick win. Check the "look inside" feature on Amazon to see a couple example problems and solutions.
Unfortunately the book only has 50 puzzles. I really wish I could find more books like it.
http://www.amazon.com/Its-Your-Move-Puzzles-Everyman/dp/1857443411
Forcing chess moves by Charles Hertan is an excellent tactical book that focuses on better calculation, it won chess cafe book of the year award and personly I find it the most enjoyable book I have ever read
Computer workout?
About mathmatics?
I'm looking for ways to train one's calculation skills. I was looking around on the web, and many people suggest tactical exercises. I'm not sure this is the right way, though, since tactical exercises are usually limited to quick mates or quick material gains. Hence the mindset when doing tactical puzzles is very different from a real game.
Ideally I'd like to find a set of exercises featuring tactically difficult positions, where one has to find the best line, and where a discussion of the various lines is provided, to compare afterwards.
You might say any heavily annotated set of games will do, which is true, but I think a compilation of such problems would be even better.
Thanks!