Talents:
10-14: 5/5 Imp Corruption
15: 1/5 Demonic Embrace
16-18: 3/3 Imp Imp (For the instances you should be running right now)
19: 2/5 Demonic Embrace
20-22: Imp VW
23-25: 5/5 Demonic Embrace
26: 1/1 Fel Domination
27-29: 3/5 Fel Stamina
Level 29 Spec: http://www.wowhead.com/talent/?NVZcxhzh

Respec #1 (Level 30): http://www.wowhead.com/talent/?IV0iVMb
30: Respec to 5/5 Imp Corruption, 5/5 Imp Drain Life, 2/2 Imp Life Tap, 5/5 Fel Concentration, 2/2 Nightfall, 2/2 Grim Reach
31: 1/1 Siphon life
32-34: 3/5 Suppression
35-39: 5/5 Shadow Mastery
40: 1/1 Dark Pact
41-45: 5/5 Demonic Embrace
46-49: 4/5 Fel Intellect
Level 49 Spec: http://www.wowhead.com/talent/?NA0iVMboVo0xm

Respec #2 (Level 50): http://www.wowhead.com/talent/?NV0fVRbZcxhuMho
50: Respec to 5/5 Imp Corruption, 2/2 Imp Lifetap, 3/5 Imp Drain Life, 5/5 Fel Concentration, 2/2 Grim Reach, 2/2 Nightfall, 1/1 Amplify Curse, 5/5 Demonic Embrace, 3/3 Imp Imp, 3/3 Imp VW, 3/3 Imp Succubus, 1/1 Fel Domination, 2/2 Master Summoner, 3/5 Unholy Power, 1/1 Demonic Sacrifice
51: 1/1 Siphon Life
52: 1/1 Curse of Exaustion
53-55: 3/4 Imp. Curse of Exaustion
56-60: 5/5 Shadow Mastery
Level 60 Spec: http://www.wowhead.com/talent/?IV0fVRbkAZcxhuMho

Note about Drain Tanking:
Common knowledge is that Drain Tanking is the most efficient leveling spec. Few realize, however, that it is nearly useless before level 30, due to the weakness of the lower versions of the Drain Life spell.

How to level:
Grind and quest at the same time. Either look for multiple quests that involve killing the same mobs, or look for multiple quests in the same areas. The more quests you can get done before running back to turn in, the faster your leveling will go. Also, remember to set your hearth to new places frequently. Every time you plan to level in a new place for more than an hour, set your hearth there, as the time saved by teleporting back to the nearest down instead of running is incredible.

Leveling Guides:
James' Alliance Leveling Guide - http://www.wow-pro.com/Knowledge_Base/cat=135.html
Joana's Horde Leveling Guide - http://files.filefront.com/1_60rar/;4993624;;/fileinfo.html
Proxy/Vince's Mage Leveling Guide - http://home.comcast.net/~mxnvincent/mage.htm (Thanks Arandor for the link!)

While the last one is meant for mage, it shows some good grinding areas.

Summary:
1-9: Do your lowbie quests. Should take 2-3 hours.

10-29: Stack all your DoTs on a mob, send in the VW, stack all your DoTs on a second mob, and then wand the second mob to death. Repeat until low health, and then bandage/Consume Shadows. Each level should take about an hour, moving towards an hour and a half as you get closer to 30. If they are taking longer, figure out what the problem is (mobs too far apart, mobs have too much life, too much downtime) and move to an area that solves it.

30-39: Drain Tank: Load the mob up on DoTs, send in the Succubus, cast Drain Life while the mob beats on you, rinse and repeat.

40-49: Drain Tank, but instead of Life Tapping, use Dark Pact. A good rule of thumb is, DP only if your pet is over 50% mana, because if they run out it will slow your fighting time down. The easiest way to do this is to turn off all autocast abilities on your Succubus, and remember to DPing during the fight (rather than after) as a good time-saver because your DoTs will help keep up the DPS.

50-59: The Drain Tanking gets even better now. By Demonic Sacrificing your pet, you are able to regen even more life per fight, keeping up with increasing monster damage, while still 'Tapping up to full mana whenever you want because of the passive regen.

60: You are now specced SM/DS, which is generally accepted to be best grinding spec. Do whatever killing you need to do, grind factions etc., and then respec to your high-level build of choice.

Equipment Choice:
"I found that during early levels it was best to focus on +sta, and at around 35 or so to switch to +shadow or +dmg. If you are specced into affliction and are draintanking, +dmg will be vastly superior to +sta. You kill the mobs much faster for the same mana, and heal yourself that much faster." -Clutchsama

It's my personal opinion that up until about level 40 you should stack +sta and "of the Eagle" gear. At 30, start looking for Shadoweave gear, and around 40 switch into it. Full Shadoweave can be made by tailors, and will give a bonus of +114 shadow damage, along with some Int and Sta, and can be equipped at level 44.

In general, while using Dark Pact I stack as much +damage gear as possible, and while using Demonic Sacrifice, I stack as much +sta gear as possible, while keeping a minimum of +100 dmg.