#1: Do not use mana on minions
#2: Do not use mana on harassing champions that you have no chance in killing (Example: Alistar)
#3: Do not ever let yourself get to a point where you're empty on resources unless you're planning on going back, or you just duked it out. Always, ALWAYS have enough to blow your load on an opponent at any given time.
Exceptions are when your resources come back faster than you spent them, or it is in your best interest to spend those resources (Like Cass), or when you're planning on recalling soon, or when someone is about to pop clarity. You get the idea here. Save mana for champs, last hit minions with auto-attack, do not harass/poke against champs unless you're sure you're going to put them at a disadvantage.
Energy champs can frequently dump their energy that can be slow to recharge. Champs like Kennen/Akali/Rumble/Lee Sin/etc should keep a relatively high energy pool if they're expected to charge a player at a moment's notice. For example, don't charge with an R with Akali if you have 40 energy.