The first post outlines pretty much everything you need to know about cooldown management and how the rotation works.

The important basics of what's there is also in this abbreviated visual guide I made a while back.

The basic advice is:
Keep everything not named Geirskogul on cooldown 100% of the time.
Delay B4B casts until halfway through the gcd spin to get 9 gcds, rather than 8.
Make sure you're using 3 Geirskoguls every minute. You get 5 4th hits every minute, so:
-- 1st gsk between first and second 4th after a cast of BotD
-- 2nd gsk between third and fifth 4th after a cast of BotD - more than 2 gcds prior to the fifth
-- 3rd gsk after fifth, but before you hit the BotD - usually it goes 2nd>gsk+botd>3rd>4th (combo hits)

If you think you're doing all of this and still coming up short, my advice is to record a run and judge yourself. Check cooldowns and see if any hang for extended periods and such.