One of the most common mistakes in playing speed black involves people trying to play it with a control black mentality. Whenever playing speed black, there is one simple thing to keep in mind- get a big guy out and start swinging. To illustrate this point, this is the deck I played at regionals (based largely off a Price design): Deck Outline:

• Fast Mana: Dark Ritual, Blood Pet, Peat Bog
• Fast Creatures: Phyrexian Negator, Hidden Horror, Lurking Evil, Skittering Skirge
• Disruption: Discard, Tangle Wire, Rishidian Port
• Creature enchantments to make the creatures bigger

THINGS THIS DECK IS NOT!

1. Good against opposing creatures
2. Consistent
3. Strong mid-late game

All in all, speed black does one thing and only one thing- overwhelm other decks by getting explosive draws. Under certain draws, speed black can literally kill the opponent before they have the chance to play a single non-land permanent. If however you find yourself playing against too many creature decks, try experimenting with some "gasp" control black elements, such as Masticore or a more Vampiric Tutor-based construction (Finkel's US Nationals deck is an excellent reference). Don't forget to have fun while watching your helpless opponents :).

It is very easy to get somewhat caught up with the idea that while playing the speed black deck you'll ALWAYS win by turn 4, but that is simply not the case. The speed black deck tends to be very inconsistent. Sometimes the hand might open up to be swamp, swamp, ritual, ritual, skirge, twist, duress, and other times it looks more like port, swamp, skirge, unmask, tangle wire. blah blah. The entire point of the deck is to get a huge monster, close your eyes, and just keep on swinging. Unfortunately, the oppenent also tends to like to do things like play creatures, which this deck HATES. Ritualing out a negator is not very strong when the opponent drops a turn two albino troll. Furthermore, if the opponent somehow manages to kill your monster...such as "I'll shock your twisted skirge," you're in trouble. They've essentially wasted three of your cards (dark ritual, skirge, twisted experiment) with one of yours, and you probably won't be able to follow up with another beast (look above to read about how this deck is also called mr. inconsistency). All and all the deck is very very situational. It does very well against a field full of combo decks and other decks that cannot do anything to stop those Negators, etc, but it really really really falls apart against most other things.