Swim Smooth have some training session books with a series if swim sets, based around using CSS.

+1 to this.
Swimsmooth have lots of information on this, but in essence you can think of it from a training perspective a bit like threshold pace in running.
I swim three times a week with a swim smooth squad....
1 technique session with no focus on CSS pace
1 threshold session with lots of work at, slightly above and slightly below CSS pace over relatively shorter intervals (50m to 400m)
1 endurance session with work mainly at about CSS +3/+4 (ie pace per 100 plus 3 or 4 seconds) over longer intervals (eg the other week as part of a 5 km set we did 2 1000m intervals on CSS +3)

Think of it a bit like running, you can't sustain threshold pace for long so you do shorter intervals at or above that pace and longer endurance intervals below that pace.
Rinse, repeat and re test every 6-8 weeks to see how you are improving.