The funny thing is that you concentrate best when you're
not trying to. It's one of those classic contraditions.
The worst thing is realizing how much work you've just been
doing, and how smoothly it was going, only to have trouble
getting 'back into the groove'.
An interesting read is "How To Meditate"
which is a very broad overview of the subject. Instead of
focusing on any one religion and its methods, there is a
discussion of the historical initiatives and techniques
of most popular beliefs.
There is no "How to Be a Zen Master in 21 Days", or
"Enlightenment for Dummies". Like learning programming,
these things take a long time to master.