Fiddlesticks is a pretty awesome guy, but he's a great jungler with amazing gank potential. In this guide I'll try to convey some of the things I've learned jungling with this man composed of sticks, straw, and concentrated badassery.

For runes I take the following.
Greater Mark of Insight - These are pretty much standard on all casters, and since the creeps in jungle got buffed so they actually have MR, these are so much more necessary.
Greater Seal of Resilience - Gives you the armor you need to survive those first few creep camps. I have tried playing around with runes to leave this spot to Greater Seals of Clarity, but I haven't found a way that works. Feel free to share in comments, if you do.
Greater Glyph of Focus - The CDR allows you to get your drain off more, and as a result, not die to the creeps.
Greater Quintessences of Potency - The 15 level 1 AP really helps move things alive.

These are what work for me, but if you have other suggestions, again, feel free to share.

AP Jungler Masteries, 9/21/0 getting the smite mastery, and AP/CDR buffs. Pretty simple.

I start with amplifying tome and a health potion. Starting with a tome helps speed things along with your build, as opposed to dropping cash on a Doran's.
I follow that up with Fiendish Codex, mostly for the mana regen. Fiddlesticks is HEAVILY mana-reliant, and if you don't have blue, you'll at least need a little bit of something to keep you going.
Sorc boots are recommended, as he's a heavy caster, but Mercury Treads are fine if the enemy has heavy CC, so you don't get stunlocked without being able to toss in your drain.
Morello's Evil Tome, for the increased mana regen and CDR.
I then get a catalyst, to start the beefiness train rolling. I build Banshee's, as opposed to RoA. Because when you ult into a fight, chances are, you'll have some CC flying at you. This will give you the time to drop your fear/drain on an enemy.
I get Abyssal Scepter next, as it's pretty much core on Fiddle. The AP give you damage, MR gives you some more layers of beefiness, and the passive works great with fiddle's passive, since you'll probably be right next to your target most of the time.
Rylai's is great for the health/AP combo alone, but having a massive AoE slow, a slow on your drain, and slows bouncing around with your Dark Wind, is absolutely amazing. Sure the slow isn't much, but that's 15% longer that some poor sap is stuck in your drain.
Rabadons is that final chunk of AP that'll carry you through lategame without becoming absolutely useless.

Other items can be subsituted into this section:
Rod of Ages - If you absolutely adore this item and refuse to build banshee's, take it. It's a matter of preference. The extra AP isn't really necessary, but it does have a great passive.
Spell vamp items, like Will of the Ancients - Rushing up a Revolver instead of Codex is a risky move. If you don't have Mana Regen runes, you might find yourself Out Of Mana when you go into a fight. It really helps your drain, sure, but drain heals plenty as it is.

Generally, I run R>W>Q>E, taking a level in E at level 2 to help your jungle.
Terrify - A great spell, grab this at level 4 absolutely. This is your only heavy CC and a damn strong one too. Without this, your early ganks are nothing. Considered OP by many, since it lasts a maximum 3 seconds.
Drain - Your bread and butter, combined with Terrify. This is an amazing single-target spell. While they're running aimlessly, or CC'd by a team mate, you're sucking out all of their life like an evil, stick-built mosquito.
Dark Wind - A lot of people max this before Terrify. But since it no longer increases the number of bounces with each level, its main use is for the silence.
Crowstorm - CAW. CAW. CAW. Your ultimate is the biggest "F*** YOU" to the enemy team. Cast this from a bush, behind a wall, anywhere where they can't see it coming. You now have a massive AoE damage around you, slapping the enemies in their collective faces with your mighty birds. Fear someone once you've ulted in and start draining. That person is almost guaranteed dead.

BEST SPELLS
Flash - Ult, Flash. You just jumped farther than sight range. You can ult from anywhere now. You're like a mexican jumping bean made of death. (Especially if you use Bandito Fiddle) Also great for escapes and your first level 4 gank. According to the current meta, you should never NOT have flash.
Smite - Don't jungle without smite. Just don't. It's slower, leaves you wide open to steals, and just sucks. If you jungle without smite, you don't deserve a birthday party.

OTHER SPELLS THAT ARE PRETTY OKAY
Exhaust - Keep fools in range of your draining and ulting more. Also is a great deterrent to pesky melee DPS champions, like Tryndamere or Master Yi.
Teleport - Allows you to quickly get to a tower or fight in trouble, or teleport to a hidden ward to set up your ult. Teleport is more of a laning spell, so it's not the best for Jungle Fiddle
Cleanse - CC'd during your ult? Don't want dem fools to be escapin? CLEANSE DAT. Somewhat unorthodox, but it could be useful. This build builds against CC anyway, so it's not really the best to use.
Ignite - Can be casted during your Drain, so adds that much more to your single target damage. Maybe the best choice out of the "other' spells, if you hate flash for some reason.

Start at blue buff, with an amplifying tome and one pot. Leashes are awesome, but not always necessary. Start by auto-attacking the golem. Wait for it to hit you twice, then drain. The second drain ends, use your potion, then keep draining when it's off cooldown. Smite to finish it.
DO NOT USE YOUR POTION DURING DRAIN. IT WILL CANCEL THE DRAIN AND YOUR *** IS TOAST.
You should leave blue, after killing the lizards, with near full health. Head to wolves. Kill the wolves. You should get to level 2 either from the Blue camp or the first wolf you kill. Grab a level in Dark wind at level 2. Feel free to spam it here.
Go to wraiths, start by Throwing E at the blue wraith, then draining blue wraith. Finish off the wraiths.
Straight to red from here, Drain the red buff. Do not use your Dark wind, you won't have the mana for it. Just keep draining while it's off cooldown, and smite it to kill.
Move up to double golems, simply drain them down, you want to conserve mana, as your blue is almost gone. At level 4, grab a point in fear, then head to either the side lane or middle for your first gank. Let the lane push a bit, then run, flash behind your target, then Dark Wind, Fear, Drain. Hopefully your lane mates there will toss their CC and damage and ensure a kill. You should recall after this first successful gank, then clear up the jungle and gank as necessary.
At level 6, you should gank again. Your flash will not be up, but your Ult>Fear>Drain combo will easily get a suprise kill.
With blue buff, you can solo dragon definitely at level 7. It can be done at 5, but it is riskier.

First build, anything missing/should be added, let me know in the comments, and I'll add it as fast as I can. The things posted here are just preference, and I'm open to suggestions, and I'll try your ideas out and update this build with those ideas.