DO NOT buy the typical jungling items. I've tried them and they do not work well with your spells, so I don't think it's really efficient to buy for an example Hunter's Machete.
Other mistakes I see a lot, is that people playing Fiddles waste their money on HP pots (or Mpots) and I really don't understand why. If you get low HP; drain, if you have mana problems; switch your runes until next time, don't waste all your spells on creeps and get the blue buff.

To be efficient in your jungling you need to get the assists from your ganks (don't go for the kill, then every1 will start hating on you and steal your jungel, and especially your blue buff). To get this you need the laners to kite the enemy near either our turret or near the bush so you can ult. Once you ulted, you throw a dark wind on them, then you silence one of them and start draining him. When drain is up, or interrupted (champions such as Annie can do that so watch out for that if you're ganking her), you throw another dark wind.
^ This is ONLY if they are not hardcore melees or tanks. If they are, then ult, and use your active form Zhonya's. Let the laners do the job, and when the active goes ut, you finish the combo as I explained over, by dark wind - fear - drain.

I have played Fiddlesticks for such a long time now, a lot the play seems obvious to me, so if you have any questions, I'll be glad to extend the guide. I'm a g1rl and this is my first guide here..so.. Just leave me some feedback, K? :D