It's about reacting to what your opponent does and/or preventing them from reacting to what you are doing so:
You'll have to first get comfortable with your class. Learn what do ALL your spells, talents and skills do, your strengths and weaknessess. That way you'll know what important cooldowns you have at your disposal.
After awhile you'll form your own strategies versus certain classes (most of them get predictable after awhile)
We can help you more if you can be more specific, like... what class+spec vs what class+spec.
If you play on Icecrown Alliance, I can help you with learning how to deal with Death Knights.
Also there arent even rotations in pve, never ever use that word.
Well, scorch spamming TTW mage will always lose to fb spamming TTW mage, so there is a rotation in pve which is giving u max potential... But I do kinda see what you mean :)