A couple helpful tips when playing fiddle support.

1) You can use your birds to help your ADC cs, but only do so at the beginning of minion waves. Stealing your ADCs CS the the easiest way to piss off your ADC and set them behind

2) A lot of people don't know how to react to a fiddlesticks support, therefore, YOU ARE SCARY. Capitalize on that if you can.

3) People run away from you. If your two enemy laners are running away together and they aren't super close to minions, that is the perfect time to throw your birds. The birds will bounce off of each of them repeatedly, doing MASSIVE damage and a lot of time, provoking Thunderlords. It is such a satisfying feeling and a lot of times, can secure kills that your ADC might not be able to get. (Make sure you aren't kill stealing though as that is very easy to do as fiddle)

4) Ulting without kill-stealing is a pretty difficult task at the times. Usually you want to wait until they are about 1/2-3/4 health each and MAKE SURE YOUR ADC IS READY TO FOLLOW UP. A bush is the best place to ult from as they cant see you channeling it. Its a good idea to ult behind them and fear the ADC towards your ADC to make sure they get the last hit, the support will usually try to stick around to save them giving you extra time to get both of them. Try to take notice of how well your ADC is at last hitting minions because they will need to do the same thing to the enemy champs in order to avoid you taking the kill. Rylai's is your best friend when you ult.

5) Ult->flashing. DO NOT BE AFRAID TO DO THIS. A lot of times the extra ground you gain from that flash is vital to getting the enemies inside your ult. It is always better to Ult->flash from a bush than to channel your ult in sight of enemies.