Some rogues, including myself, feel the tendency when fighting PvE targets to use certain cool downs at the start of a fight against a PvE target, thinking "The sooner I use Shadow Dance now, the sooner I'll be able to use it against a target when the cool down's done." Yeah, that's true, but these abilities can be used for special occasions that'll either allow you to become much more powerful than you were, or will let you escape something that you normally would have died from. Let me explain... As a subtlety rogue, until you hit level 80 and get our Fan of Knives AoE, we can only attack one target at a time. So, our goal is to attack one target at a time and move on to the next (hopefully from stealth). But for the times when we get two or three targets on us at a time that we weren't prepared for... we still can only fight one target at a time, and the only combo point ability that can carry over from target to target is Slice and Dice. If you saved some, or all of your special abilities for a situation like this, you can deal with it better.
Evasion is great when being attacked by multiple targets because it'll help you avoid gettin' hit on until you can finish one of your targets and move on. Blind is amazing because it allows you to keep a second target outta' the fight for 10 seconds. Shadow Dance is ideal because it'll allow you to do much, much more damage to finish one of your targets to move on to the next. Sprint and Vanish are there to help you escape the action (or kill one or most of your targets and escape the rest when you can't take any more hits). And Preparation is good clearly because it gives you an extra Sprint, Evasion and Vanish (in other words, finishes their cool downs). Rather than using them whenever their cool down is up, by saving them it'll make you stronger and more survivable in the long run.
This also goes for PvP. It's pretty much the same example (besides arenas, when it's usually better to blow all your cool downs on one target to try and kill it early on). Let's say you're in a battleground, fighting in AB. A target comes along, such as a mage, that you think you can kill without dying. Shadow Dance and Evasion are there, just waiting to be used. Do you use it? No... save'em if you can. You end up killing the mage with 50% health left. Good. Now you see a holy priest healin' that no one's attacking. This will be easy. You stealth, Shadow Step and do your thing. To your surprise, the priest is barely gettin' hurt and is healing themselves while you're losing health. "Crap..." your thinking, this wasn't what you expected. What do you do? You use those abilities you saved on this target; Shadow Dance and Evasion. You know realize "Man, I'm so glad I saved these abilities for a situation like this." That's what you gotta' do.
There's times when you not doing anything special... 'yer chillin' in Stormwind, just takin' care of business 'n' crap. You gotta' get through a long building to mount up. You could activate Sprint to get outside to mount quicker. Don't be lazy; save it. In times like these, nothin' special will happen... but on those times when you hear somethin' and go "Crap, an enemy player's near me in my city!", don't start fightin'em, have them kite you with you thinking "Crap, you're lucky I wasted sprint running to the bank...". Point is; you never know when you'll need these abilities and you'll be doing yourself a favor not blowing them if you can.
These seems like complete common sense to me, but clearly it's not to some rogues because I see sprint and vanish and evasion and others wasted for the simplest (and stupidest) reasons, both in raids, PvE and PvP. Then when it comes time for the serious stuff, those rogues get beat down or shut down while I'm still killin' 'n' thrillin'... simply because my abilities are ready and theirs weren't.
Do yourself a favor and be smart about your abilities.
Jeaxx