Jumpy servo with arduino
I can reproduce the nervous servo with the soft pot and flex sensor. In case of the soft pot I can only make it steady when I hold my finger steady on the soft pot. If I don't, the servo starts to wiggle. 

Adding a small capacitor (I added 100nF or 220nF) between ground and pot output (middle pin) stops the wiggling. Your simply picking up static.

Keep wires to the soft pot close to each other, don't make big loops in between them. You could braid them together for example. This and the small cap will reduce the picked up static and with that stop the wiggling.

The soft pot has an extremely high impedance on its middle pin when it is not touched. I guess it is only makes contact to the resistor when actually being touched. This explains why the servo gets nervous when the soft pot is not used. The middle connection is basically a loose wire &rarr; antenna and the analog input of the microcontroller is very sensitive / high impedance.