What the others said about clean... It needs to be clean. I've used a tourch and iron. I prefere the iron. Been using plumbers solder. HAs some silver in it, and it's as cheap as rosincore solder, and just as easy to use.
What are you guys using to nutalize the acid flux? I clean it really well after wards with Denatured Alcohol, but sometimes still get soem rust.

Also if you choose to use a soldering gun...(and nothing wrrong with it), just make sure your servos and any other electric motor are no where near the gun. With the gun you are shorting a winding in a transformer ( basically), and this generates enough of an alternating magnetic field to, and can (and will) demagnatize the magnets in a motor.

I've never had this happen, but it did to a buddy. A known good servo no longer worked after using near a soldering gun.