Your gear will determine about 70% of your feedback. Shitty pickups, ones that haven't been potted properly, tend to give off loads of feedback. Although some times uncontrollable! Gain!! Lots and lots of gain......if you want feedback, you gotta have gain.
Here's a little trick to produce feedback on command. It does , however require extra gear....very cheap all the same. First get a 1/4 inch(male) adapter that spits off into 2 1/4 inch(female). You can buy this at any radio shack for less then 5 bucks. Take one of the 2 and hook it up to your rig like you normally would. The spare female 1/4 plug in......take this one and hook it up to smallest battery powered amp you can get your hands on. I use one of those little cigarrete pack amps.
So now you've basically hooked up to 2 amps from the same guitar. Whenever you want feedback just pick up the smaller amp and bring it in close. Move it around to maipulate the sound, you'll be amazed. This also works at lower levels...but not too low. Finally, make sure your using lots of GAIN!!