Your best bet is a power aura or weak aura. There are some spell flash addons out there but I've never used any. WA does pretty much everything for me.
I use OmniCC as timoteo suggested, and I would recommend this one for pretty much every class. You can set the minimum threshold to whatever you want, so it will only show countdowns for cd's longer than X seconds.
You can also use either a weakaura or tellmewhen to put an icon up on your UI when HS is available, or make a countdown timer to show seconds until the cooldown ends.
Weak Auras is nice because you can pretty much set it however your poison. Like bars, it has that. Big ole icon with a CD, it has that. Old Power Auras animations, it has that as well.