This is a technique I developed for my lock. It is reckless and can get you killed if done wrong. But with some skill, you will NEVER run out of mana, no matter how little of it you may start with. Using this technique allows you to kill enemies nonstop without having to ever sit and drink. This will save you literally hours when you add up all the time you spend drinking over the course of 70 levels.
First of all, you can do this with any spec, but it will be SO much more effective if you go affliction and get the talents I recommended in my other post. Secondly, the more shadow damage you gear yourself with, the more effective this will be. Your health and mana are relatively irrelevant when you use this technique, so I recommend using nothing but shadow damage on your armor.
So say you have an enemy you want to kill. If you have a void out, have him start beating on the enemy before you
pull so he can hold aggro long enough to get 4 DOTs on him before he
runs at you. Start as far away as you can get and pull him by using unstable affliction. You want to start with this because it is the only non instant cast DOT you have. Drop corruption on him, then put a curse of agony on him. Then lay a siphon life on. At this point he will have 4 massive damage over times on him tearing up his HP. The beauty of this is, each DOT increases the amount you drain from siphon life.
Once you have all 4 DOTs off, use a life tap to drop your hp and fill you up to full mana. Then use drain life. No matter how much your void is tanking up to this point, the mob will almost definitely charge you and not back off. But with 4 DOTs on it, your drain life will do a huge amount more damage. It takes about 3 seconds for me to fully drain a mob's HP and recover myself completely from half health. Usually he will drop dead before he gets near me. So I end the battle with full HP and full mana.
My lock has good enough shadow damage to kill so fast I don't have to bother tanking. I just DOT him with agony and corruption, life tap, then drain life and he will die before touching me. I usually kill 2 mobs at a time with this method. In instances, I have actually done this and tanked better than a warrior. The life drained is so extreme that I heal myself usually faster than the enemy can hurt me. I don't recommend trying this though.
This technique constantly balances your HP, Mana, and the enemy's hp. Your mana goes down first. Then your health goes down to put your mana up. Your mana goes down just a touch to fill your hp up. The enemy's HP always goes down and fills you up. If you do it properly you can always come out of every battle with full HP and mana and just move right on to the next.
Recovering from low HP/Mana: If you aren't quite as effective as my lock, you can still do this. Your hp/mana will deplete very slowly. When it gets low, set your voidwalker onto an enemy. Set just a siphon life on it and life tap yourself down to really low hp. This will recover most of your mana. Then siphon life until your HP is full. A good void should be able to hold aggro in this situation. Then just tap again, and drain away. Keep siphon life on at all times as it will keep healing you. After a couple times, the enemy will die and you will end with full hp/mana.
Last Resort: If all else fails and you get in a tough situation, are stuck in combat, or just feel like doing this the easy way, you can use dark pact to suck up all the mana from your pet. I use this only as a last resort because once your pet's mana is depleted, it will only recover slowly as it naturally regenerates. The point of this all is to keep a constantly flowing mana supply.
Other Last Resort: You can almost instantly recover mana in a pinch by using life tap to drop you down, then firing off a death coil to fear the enemy and recover the HP right back. As he runs, charge up another fear and send him running further. Then you can either run away yourself, or life tap again, dot him up, and eat his HP back up. I use this a lot during gankings.
Other Other Last Resort: You can always use a quick life tap/health stone or potion to jump back to full mana. By the time I get this desperate, I have usually finished off 5-10 enemies.
Final Resort: If all else fails, use howl of terror to send your enemies running and run away.