Keep in mind that your passive Dread will be seen when your enemy is close enough, even when they lack vision of you. This can occasionally ruin potential ganks in which the enemy gets the debuff and retreats, so make sure that your passive is out of range until you engage.
Dread is very useful early game. If you are laning with a mage or caster, this will allow you to push your lane much better.
Terrify with Drain is an amazing combo that can turn the tables in a fight. To make sure that the enemy stays within range, Terrify them against a wall, where they will be under heavy fire and under control.
If you use Drain on enemies that go into stealth or flash, Fiddlesticks won't stop channeling until they go out of range. Drain also continues through Sanguine Pool, Rebirth and Playful.
In early game lane, Dark Wind is an extremely effective poke. With no ability power runes it does 65 damage a bounce with no reduction in damage from successive bounces, and it only costs 50 mana.
Furthermore, Dark Wind can be used from an extremely long range since you can bounce it from melee minions to range minions to the target character. Even at closer range it silences so you can escape often without taking damage. Since Dark Wind is the last ability you should level, use it early (levels 1-5) and often to give you a lane advantage. At middle levels (6-11) this ability will be far less effective at poking from range and clearing minions, while champions with long range pokes will stop you from farming.
When taking down opponents, consider using him as an assassin, as Fiddlesticks shouldn't take on many champions head-to-head. Sneaking up on an enemy champion then using Crowstorm, Terrify, and Drain maximizes damage. Dark Wind is most useful for silencing enemy champions but can do very high damage to bunched up enemy champions (especially pairs of champions with no minions around as the spell will bounce between them until it reaches 5 bounces, keeping them both silenced for the duration). It can also be very effective at farming minions as it deals 50% bonus damage to minions. You can also initiate a fight by using Flash or Crowstorm over a wall and using the aforementioned combo onto the enemy team.
Starting items for Fiddlesticks should almost always include some form of mana regen. A few Mana Potions or a Doran's Ring will allow you to throw out pokes constantly and keep your life up with Drain.
Zhonya's Hourglass is a must have on Fiddlesticks. It gives you the ability to very effectively tower dive or initiate team fights with Crowstorm or block the massive amounts of CC and incoming damage that will be thrown at you whenever you choose to Crowstorm in.
Often times it is best to rush Zhonya's Hourglass since Terrify -> Drain combo will give you plenty of sustain while Crowstorming in for 1v1 assassinations, and what you really need is early team fight sustain because you often have to get into the mix to do damage.
Buying Rylai's Crystal Scepter will maximize your damage with Drain, as your enemy is slowed when trying to escape. It will also help your ultimate to maximize damage and also be an AoE slow in large team fights leading to advantage.
If the enemy team has a strong invade, such as Blitzcrank or Alistar, consider starting with Boots of Speed, Sight Ward and a Health Potion. Placing the ward at either your blue, or the enemy team's blue before the jungle camps spawn; this can give your team valuable information on where the enemy jungler is starting and make it easier for you to steal their blue if they're invading your side of the jungle.
Fiddlesticks deals most of his damage from his Drain and Crowstorm and both have channels which can be interrupted. This means that hard CC, like a stun, can stop most of Fiddlesticks' damage.
A common Fiddlesticks strategy is to hide in the brush and wait for an unaware champion and use his Crowstorm. Good warding and map awareness can stop Fiddlesticks from ganking you from typical locations.
If you notice you have the Dread debuff, but you can not see Fiddlesticks, he is probably hiding nearby in a bush or behind a nearby thin wall. Use this to your advantage.
When starting a team fight try to save your ability with CC that can interrupt channeling abilities.
If you see an enemy Fiddlesticks channeling his Crowstorm, use a CC ability that can stop him (stun, silence, knockback, knockup).